GST Invoice Mismatches: 8 Things to Check Before Filing GSTR-1

A mismatch between a supplier's GSTR-1 and a buyer's GSTR-2B is one of the leading causes of ITC disallowance in GST assessments. The mechanism is straightforward: GSTR-2B is auto-populated from the counterparty's GSTR-1. If the supplier's GSTR-1 does not reflect the invoice correctly, the buyer's GSTR-2B will not reflect it either, and the buyer loses ITC until the supplier corrects the filing.

For CAs reviewing GSTR-1 before submission, a pre-filing checklist reduces the risk of generating mismatches that hurt both your client's buyers and your client's own compliance record. Here are the eight areas to review every month.

1. GSTIN Accuracy in B2B Invoices

Every B2B invoice must carry the recipient's correct GSTIN. A single character error in the GSTIN means the invoice will not flow to the correct buyer's GSTR-2B. The buyer will not receive ITC, and they will escalate the issue to the supplier, who must then amend the invoice in the next GSTR-1.

Check that the client's accounting software has up-to-date GSTIN records for all buyers. GSTIN validation can be done via the GST portal's taxpayer search at gst.gov.in. For large transaction volumes, CBIC provides a bulk GSTIN search API that can be integrated into accounting tools.

2. Invoice Date vs Tax Period

An invoice must be reported in the GSTR-1 for the period in which it was issued. If the invoice is dated 28 March but the client attempts to report it in the April GSTR-1, it creates a mismatch. The buyer will expect the invoice in March's GSTR-2B but will not find it there. If they file March's GSTR-3B claiming ITC on this invoice before the supplier reports it, the ITC will be disallowed on matching.

Under Rule 59 of the CGST Rules, an invoice for a tax period cannot be reported in a GSTR-1 for a later period if the GSTR-1 for the correct period has already been filed. Cross-period reporting is therefore only possible when the original period's return has not yet been filed.

3. Taxable Value vs GST Amount Consistency

The GST amount on an invoice must equal the applicable rate multiplied by the taxable value. This sounds obvious, but rounding errors in accounting software, or manual invoices prepared by clients, regularly produce mismatches. A GST amount that differs from the rate-times-value computation by even a small amount can cause validation warnings during GSTR-1 filing and, more importantly, creates a discrepancy between what the supplier claims to have charged and what the buyer claims to have paid.

Verify that IGST, CGST, and SGST amounts are separately stated (not lumped together) and that the sum of CGST and SGST equals IGST where applicable. Interstate and intrastate invoices must use the correct tax type.

4. HSN/SAC Code Accuracy and Reporting Thresholds

Effective 1 April 2021, HSN reporting in GSTR-1 is mandatory at varying digit levels depending on turnover:

• Turnover up to Rs. 5 crore: 4-digit HSN for B2B; optional for B2C
• Turnover above Rs. 5 crore: 6-digit HSN for all invoices

An incorrect HSN code affects classification for the buyer and can attract scrutiny during audits. HSN errors in GSTR-1 also create downstream problems in e-way bill generation where HSN is referenced.

Review the HSN summary table in GSTR-1 before submission to ensure total taxable values by HSN match the individual invoice values reported. The portal will flag gross mismatches, but subtle errors pass through.

5. Place of Supply Correctness

The place of supply determines whether a transaction is treated as intrastate (attracting CGST and SGST) or interstate (attracting IGST). An incorrect place of supply leads to the wrong tax being charged, which cannot simply be adjusted in the next period. It requires a credit note and re-invoice, or an amendment in the subsequent GSTR-1.

Common place-of-supply errors include:

• Services where the billing address and the service delivery state differ
• Online transactions where default place-of-supply logic in software differs from the legal rule
• Export transactions where the place of supply must be correctly coded as outside India (9Z)

Verify the place of supply for any transaction involving a buyer in a different state, for service invoices where no recipient GSTIN is available, and for all export invoices.

6. Credit Notes and Debit Notes Linked to Original Invoices

Credit notes and debit notes must reference the original invoice number and date. If a credit note is reported in GSTR-1 without the correct original invoice reference, the matching system cannot link it to the original transaction. The buyer's ITC reversal obligation under the credit note is then not triggered automatically, which may lead to ITC excess claims on the buyer's side.

Additionally, a supplier can issue a credit note in GSTR-1 only up to 30 November of the year following the financial year of the original invoice, or the date of filing the annual return, whichever is earlier. Credit notes beyond this window cannot be reported in GSTR-1 and must be handled differently in the accounts.

7. Nil-Rated, Exempt, and Non-GST Supplies Separation

These three categories are often lumped together in client records but must be reported separately in GSTR-1. Nil-rated supplies attract 0% GST but are taxable supplies. Exempt supplies are those excluded from GST under Schedule III or specific exemption notifications. Non-GST supplies (like petroleum products) fall outside GST entirely.

Incorrect categorisation between these three affects the client's aggregate turnover computation, which in turn affects eligibility for composition scheme, QRMP, and various exemptions. It also affects the denominator used in ITC reversal calculations under Rule 42. For more on ITC reversal rules, see ITC Reversals Under Rules 42 and 43.

8. E-Invoice Reconciliation for Eligible Suppliers

For taxpayers whose turnover exceeds Rs. 5 crore (the current e-invoicing threshold), every B2B invoice must be generated through the IRP (Invoice Registration Portal) and carry a valid IRN (Invoice Reference Number) and QR code. The IRP pushes e-invoice data directly to GSTR-1. However, this auto-population is not always perfect.

Verify that:

• All B2B invoices for the period have been registered on the IRP and carry valid IRNs
• Auto-populated GSTR-1 data from IRP matches what the client's system shows
• Any invoices where IRP registration failed (due to network error or validation failure) have been re-registered before the GSTR-1 filing date
• Cancelled IRNs do not appear as valid invoices in the GSTR-1

IRN mismatches between the client's records and the auto-populated GSTR-1 are a frequent source of errors when clients switch to e-invoicing or change accounting software.

GSTR-1 vs GSTR-2B Cross-Check

After filing GSTR-1, verify that GSTR-2B for the next period reflects the expected data for your client's key buyers. This is particularly important for clients on the QRMP scheme who use the Invoice Furnishing Facility, since IFF uploads for months 1 and 2 feed into buyers' GSTR-2B independently of the quarterly GSTR-1. For a full explanation of IFF and its interaction with GSTR-2B, see The Complete QRMP Scheme Guide.

Also cross-check your client's own GSTR-2B before filing GSTR-3B. ITC claims in GSTR-3B should not exceed what appears in GSTR-2B unless you have a documented basis for the difference. Excess ITC claims create notices under Rule 86A and Section 74.

What to Do When You Find a Mismatch

For mismatches discovered before the GSTR-1 is filed: correct the invoice data in the client's accounting system and re-upload.

For mismatches discovered after GSTR-1 is filed: use the amendment facility in GSTR-1 of a subsequent period (Table 9A for amended B2B invoices, Table 9B for amended credit/debit notes). Amendments can only correct previously reported invoices; they cannot add invoices from more than two financial years ago.

For mismatches that the buyer has raised against your client as the supplier: issue a corrected invoice or credit note promptly. Delays create ITC disputes that can affect the business relationship.

Filing Timeline Reminders

GSTR-1 due dates for the current financial year are:

• Monthly filers: 11th of the following month
• Quarterly filers (QRMP): 13th of the month after the quarter ends

GSTR-3B is due on the 20th (for monthly) or 22nd/24th of the month after the quarter (for QRMP, depending on state category). For a complete filing calendar for FY 2026-27, see GST Filing Deadlines for FY 2026-27.

Filing GSTR-1 after the due date attracts a late fee of Rs. 50 per day (Rs. 20 per day for nil returns), subject to the maximum prescribed by CBIC notifications. Beyond the fees, late GSTR-1 filing delays the buyer's GSTR-2B generation, which affects their ITC claims and creates friction with buyers. Running the 8-point checklist before each filing reduces the likelihood of post-submission corrections and the reputational cost of being a supplier whose GSTR-1 is consistently error-prone.

The FiledRight rules engine automates deadline tracking for both monthly and QRMP clients, flagging GSTR-1 and GSTR-3B due dates alongside PMT-06 and IFF windows.