Note that whenever a customer returns an item to you (straight return or
trade or exchange), you do NOT edit the original sale and make the
adjustment there. This is true whether the item was purchased yesterday and
is in the current sales file or whether the item was 5 years ago and where
the sale has been archived / purged.
Instead you need to make a brand new sales ticket and record the same sku
with a -1 quantity
If there are no other items involved then proceed to the payment area to
issue a refund which could be in the form of an in-store credit slip, cash-
back, credit card refund (or, in the case of an in-house charge account
customer not a refund but a credit on account)
If the customer is getting another item in exchange, add the item(s) as
separate line(s) on the same sale. Then you can proceed to issue a refund if
you ow them or accept a payment if they owe you.
See 'Handling Exchanges' Approx page 1-23 in the manual for more details