Hi jc022
If the original sale was created as an invoice and payment was directly applied to it, then please follow the given steps to fix the situation:
1) Untick "Transactions Can't be Changed. They must be reversed" under Setup>>Preferences>>Security.
2) Go to Sales Register>>Returns and Credits tab
3) Open the credit that was created as a result of reversing the sale.
4) Delete the credit note by going to Edit>>Delete sale or simply right-clicking on the credit note after it's opened.
After you have deleted the credit note, you should be left with the original sale and its payment which can be settled as you want. Please note that if you want to reverse the payment instead of deleting it, the preference for "Transactions can't be changed. They must be reversed" should be ticked again.
Let me know if you have additional questions on this.