ERR_INVOICE_MISMATCH on QuickBooks: Invoice amount mismatch. Root cause: The invoice data does not match the corresponding purchase order, contract, or payment record. This three-way match failure occurs when invoice amounts, line items, quantities, or vendor details differ from the approved source document, triggering the accounting system's validation controls. Step 1: Identify which line item or total is mismatching. Open the invoice in QuickBooks Online. Click the gear icon (top right of the invoice) → Audit Log to see every change made to the invoice and by whom. Compare the invoice total against the linked purchase order or payment record. The mismatch is almost always in one of three places: a line item quantity, a unit price, or the tax amount. Identify the exact discrepancy before making any changes. Step 2: Check whether the product or service price has changed since the PO was created. In QuickBooks Online, go to Sales → Products and Services → find the item on the invoice. Check the current price against the price on the original purchase order. If a price list update changed the item price after the PO was approved, the invoice will auto-populate with the new price, creating a mismatch. To fix: manually override the price on the invoice line item to match the PO, then add a note in the Memo field explaining the override. Step 3: Verify the tax code is applied correctly. Tax mismatches are the second most common cause. In QuickBooks Online, open the invoice → check the Tax column on each line item. If a line item shows "Tax" but should be "Non" (or vice versa), click the Tax dropdown on that line and change it. Also check the customer's tax exemption status: go to the customer record → Tax info tab → confirm whether they have a valid exemption certificate on file. Step 4: Check for rounding differences caused by currency or discount stacking. QuickBooks rounds at the line item level, not at the invoice total level. If you have multiple discounts applied (a line-item discount plus an invoice-level discount), the rounding can produce a total that differs from a manually calculated figure by $0.01–$0.05. Go to the invoice → remove all discounts → recalculate the total → reapply discounts one at a time to identify which combination is causing the rounding issue. Step 5: For three-way match failures with purchase orders. If QuickBooks is flagging a mismatch between the invoice, PO, and receipt, go to the invoice → click "Add to PO" (or the linked PO number) → compare quantities received vs quantities invoiced. If the vendor invoiced for 10 units but you only received 8, you need to either create a partial payment or request a corrected invoice from the vendor. Do not override the quantity to match the invoice — this creates an inventory discrepancy. Step 6: Save the corrected invoice and reconcile. After correcting the mismatch, save the invoice and go to Banking → Reconcile to match it against the bank transaction. If the invoice was already marked as paid with the wrong amount, you will need to void the payment, correct the invoice, and re-enter the payment. Do not delete the original payment — voiding preserves the audit trail.