It is a bad, bad practice for retail shops (big or small) not to give the exact change to customers. Even giving a chewing gum (which is usually unchewable) in lieu of that 25 dirhams change is not acceptable at all because they are forcing the customers to buy something that they didnt want. Try paying your bill with 25 dirhams short of the total amount and they wont accept it. So it is just right to demand every dirham from them.