If you receive payment on the wrong invoice but it is the same client you can do the following to correct this.
1. Go To Customers > Commands > Adjust Receivables
2. Type in a brief description of your action, you can elaborate in the additional memo if you like.
3. Leave the Credit Amount field blank and fill in the Offset Account field with a desired account, i.e. a suspense account.
4. Find the invoice with the wrong payment on it (If the invoice is not in the list click Load All to see it). Then enter a negative amount in the $ Applied field.
5. Find the invoice that you would like the payment to be applied to and enter a positive amount in the $ Applied field.
6. Click OK.