When I post an invoice that has an early payment discount, the discount is auto calculated and posted. How can I post an open credit to an invoice that has already been paid and have the system calculate and post the discount now not allowed on the paid invoice.

Credit Notes cannot automatically detect that an invoice was paid with an early payment discount taken and subtract this discount amount from the detail distribution. Selecting the Invoice number as the Original Document does not load any information from the invoice into the credit note batch entry.

You can determine if an early payment discount was taken by inquiring on the customer (by clicking the Customer button while entering the Credit note), and drill-down to the invoice being credited. The grid in the middle of the screen will show all applied transactions to the invoice, including early payment discounts. Then adjust the detail distribution amount on the credit note accordingly.

Are you wanting to post a credit note against the invoice, or are you wanting to reverse the payment? If the latter is the case, then you can create a negative Cash payment to the invoice in question, and edit the payment details to set the early payment amount as negative as well. Select the Show All option to list paid invoices within the grid, provided the invoice has not been cleared to History.