You sent this customer an Invoice for $110 when it should have been $100.
1. First thing you need to do is to issue them a Credit Note for the difference of $10/-. Note that the customer is only going to pay you the Net $100/- for this 2 documents. Prior to entering the customer receipts, your Debtor reports will show the Invoice for $110 and the Credit Note for -$10 .
2.To knock-it-off, click on the New Receipt Button. Select the customer and enter a Chq Number if applicable. Select the appropriate Bank Account under the Account field. Enter the value of $100.00 in the Total Received field. Against the Invoice for $110, enter the amount of $110.00 in the Applied Amt field. Against the Credit Note for -$10, enter the value of -10.00. Click Save & Close to accept the transaction. Your Debtors will now be fully knocked-off.