Not sure if you've done what Matthew suggested yet, but I have additional comments to add that makes the "void" easier. Definitely make sure you change the interest rate to zero, but also change the number payments to 1 before you hit calculate. That way you are only creating one more voucher and you only have to have the next fiscal period open to post the scheduled payment remaining. You can then void the one payment instead of issuing a credit, if that is easier or makes more sense for you. You can certainly do the credit as Matthew suggests, but I find this way there is more room for error with using correct GL accounting.
Also, one thing that tripped me up in the past is that when changing a scheduled payment this way to "void" it, it confused me that the post button is greyed out in the Receivables Scheduled Payments Entry window. You actually do not need to post it again when you change a scheduled payment, rather, you can click on the save button and the scheduled payment is then saved with the new terms of zero interest and one remaining payment when you go to post it in the scheduled payments routine.
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