If I return something to the store to get a refund, is that equivalent to declaring the original sale a mekach ta’us, a mistaken sale, and the refund is returning the money which is rightfully mine? Or is it considered a new sale, wherein I sell the same item back to the original seller for the purchase price?
A practical difference might be if the store is allowed to refuse a refund. If it’s a reversal of the original sale, which was retroactively a mekach ta’us, the seller should have no right to refuse a refund. If it’s a new sale, the seller is under no obligation to buy the item back, and can therefore refuse the refund. (Obviously this refers to a case where the item is intact and able to be resold, where there are no Dina d’Malchusa considerations, and where the store doesn’t have a prior refund policy.)