It is true, generally, whether the agreement is signed or just verbal, although some contracts or agreements must be in writing and signed. There are two parties to the contract - the seller and you, the buyer. Each has the same right to expect the other to live up to its terms. You are asking the seller to cancel or break or change the terms of the contract - and he has the right to decide whether he will do so or not. When adjustment is dependent on the policy of the seller, be sure you understand the policy.
Some stores follow the policy that sales are final.
They will not refund money or give credit slips. Some stores follow the policy of refusing cash refunds, but allow the purchaser a credit slip which may be applied then or at some future time toward the purchase of other merchandise in the store. Some stores unhesitatingly give refunds, credits, exchanges, or return deposits. Exceptions include certain intimate articles, such as combs, bathing suits, mattresses, etc., which for health reasons and for your protection, are generally not returnable. Also, there could be a restriction that returnable merchandise must be returned in the same good condition within, perhaps, three business days for credit, refund, or exchange.