The mishna (Bava Metzia 44a) says that although paying money for an object does not effectuate a kinyan, there is still some obligation to abide by the sales agreement. This is in the form of a semi-curse known as a Mi Shepara. Besides a moral obligation that Mi Shepara engenders, there are also legal ramifications. The Shulchan Aruch (Choshen Mishpat 198:15) says that until the one who backs out accepts the Mi Shepara, the sale is still intact and so if the money given is lost, it is the responsibility of the seller to reimburse for it.
The Rambam (Mechira 22:3) says that if someone paid for an object that the seller had not yet taken control of himself, Mi Shepara obliges him to obtain the object to be able to give it to the buyer. The Shach and S’ma argue if Mi Shepara applies only in a case where a valid kinyan would make a binding agreement. The S’ma (209:23) says that it is talking about a case where the object is prevalent in the market, in which case a full kinyan would have been binding. The Shach (209:13) says that even in a case where there is no possibility for a kinyan to take effect, there still could be a moral obligation to uphold it, which finds expression in the Mi Shepara. He models this halacha on the gemara that situmta, a certain action that shows seriousness to acquire an object, creates a Mi Shepara even in a case where the situmta does not serve as a binding kinyan, because a serious action was done. The K’tzot Hachoshen (209:9) rules like the S’ma. The Pitchei Teshuva (209:11) says that the matter is an unsolved question.
This machloket is of great importance in our times when many financial transactions, such as sales of stock options, do not lend themselves to full kinyanim. Often also, a retailer does not have possession of the object at the time of the sale, as he obtains it from the supplier only afterward. Often these transactions are not done with the physical transfer of cash, but with bank transfers, checks, and credit card payment. These "transactions" are not more than payments of money, so that even according to the S’ma there is no more than a Mi Shepara.
Another point that requires attention is the matter of a set price, which is usually needed for a Mi Shepara to exist. One can ask regarding times like ours when commodity prices change regularly, if a kinyan works for them when the seller does not yet possess the object. The Pitchei Teshuva (209:11) says that in that case, a kinyan will not work on an item that is prevalent in the market if it is not yet in the seller’s control. (Dina d’malchuta does not exist in this area because the law does not determine that the payment of money is itself a kinyan, nor is there a clear minhag hamakom on the matter.)