When you apply finance charges in Sage 50, Sage 50 creates sales invoices with the prefix FC to designate them as finance charges. The correct method removing a finance charge depends on whether you send statements to customers. If you send statements to your customers and those customers have seen the finance charges on their statements, you will want to remove the finance charge by issuing a credit memo. In this way, your customers will see the credit on their next statement.
If you do not issue statements to your customers, yuo can simply void the finance charge sales invoice. Voiding it will ensure that you don't accidentally apply it again in the same month. If you delete it, Sage 50 will recalculate if you run the routine again.