Keeping Your Word
Question: I recently went into a shop, paid for an item and then changed my mind about buying it and asked for my money back. What is the right thing to do in such a case?
Answer: The Mishna (Bava Metzia 44a) rules that although the purchase is not halachically consummated until one takes physical possession of a portable item, the one who backs out after money has been given is reminded that "The One Who punished the people of the Generation of the Flood and Who punished the people of the Generation of the Dispersal (following the construction of the Tower of Bavel) will punish the one who does not keep his word."
Even if there was no exchange of money there is a moral obligation for one to keep his promise to give a gift or make a deal unless the loss to the one who wishes to renege is so substantial that we can assume that the other party never took the promise seriously.
There is an even loftier ethical standard mentioned in the Talmud of a Sage who felt committed to sell an item at the first and lowest price offered him while he was reciting the Shema and could not respond. Since he had decided in his heart to sell at that price, he was unwilling to accept payment of the higher price mistakenly offered by the buyer who misinterpreted his silence as bargaining.
In conclusion, keeping your word is a serious matter and a competent halachic authority should be consulted when the occasion arises for failing to do so.