I do it all the time. If customers say no when I offer them a keyring which contributes a donation to charity (because my manager pushes me to sell them because the money has already gone to charity as the store has already paid for them) I always give them change. Sometimes they give me 20 quid for a cheeseburger so i just give them a full bag of pounds and take one out as a full bag of pounds is 20quid. lol