The two previous answers are spot on. Legally you have no rights to a refund, either in full or partial. The store obviously has a policy that it will accept unwanted items back, but if opened, as in your case, it applies a charge to cover the fact that in effect the item is considered "used". The customer buying your returned item would suspect something was wrong as the box has clearly been opened, and I guess the shop will actually sell it as a "customer return" at a reduced price. So the answer is no you shouldn't complain, but count yourself lucky you are not stuck with a monitor you don't want.