Post charge for returned goods

Wife bought some trampoline parts at beginning of summer online. Main part which was the net never came. Two months later wife contacted supplier and he said still out of stock and he agreed to take back goods that had been delivered. Note these had never been opened. He called to say he was going to charge for postage and refund the difference. Still hasnt made the refund and i have told my wife to contact bank to do a chargeback. Thing that bugs me is the charge for postage, i mean we also have a postsge cost and no one is offering to pay it