You might want to discuss this with your accountant. It depends on your costing method, and your accounting method, etc. It might be easier to just make a negative purchase invoice in AP to a non-stock item created for this purpose or an adjustment in the General Ledger. If you do it in A/P you will have the correct balance owed to the vendor. The non-stock item should be set up to debit cost of goods sold and credit accounts payable (if you are on an accrual basis) since you will enter the "purchase" with a minus sign, for example.

Sage does not allow any Item transaction for a Stock Item without a quantity. Since the Item is usable but not the same, you could set up another Item and then issue a Vendor Credit Memo for that new Item with the new cost. Then Receipt your dollar return using a Vendor ID. Then through Payments apply the Receipt against the Credit Memo to finalize the process.

All that being said, you could do all of the above using the same Item ID. That would increase the item Qty at the lower cost, thus affecting that item's overall cost (depending on the costing type for this item) then do an Inventory Adjustment to remove 1 at the new adjusted cost.