Is presenting proof of a bank transfer sufficient to complete the sale, or is it considered a deferred payment (nasi'ah)? And must one accept a return of goods after they have been sold? And is it obligatory to follow up with the recipient to ensure the amount has been deposited after the transfer, or does responsibility end once the transfer is made?

The sale mentioned is not one of the deferred payment sales. The possession occurred constructively by transferring the price to the bank account, and this is considered constructive possession, recognized legally and customarily. The buyer does not have the right to annul the sale simply because the commodity does not suit him, unless there is a defect. A cancellation of the sale (Iqalah) is not legally binding, even though it is recommended. By transferring the price to the buyer's account, the obligation is discharged.