Legally, the store would be obligated to stand behind its previous manager's agreement:1) An oral agreement (sometimes incorrectly called a verbal agreement) is enforceable;2) The store manager would almost certainly have had actual authority to bind the store in this regard; and3) Even if somehow the manager lacked actual authority, she would have had "apparent authority"--i.e. to a reasonable outsider, she would have had the authority to do this--which can itself be sufficient to bind the store.Once she has obligated the store, the fact that she later is transferred,  fired, quits, etc. has no bearing on the enforceability of the agreement she entered into.In addition, if the product was defective, then the store would have to accept it back anyway, unleess you had specifically accepted the bag "as is"--merchants cannot sell you defective or nonconforming merchandise, then keep your money and refuse to exchange the item.Of course if the store will not voluntarily honor it's obligations, you'd have to sue them to get them to do so, which may or may not be worthwhile.