The Double Error
Question: I hired the services of a painter to do some extensive work in my home and he quoted a reasonable price of $3,000 for the job. When he completed his work he asked for $2,750. Aware that he had quoted a higher price I asked him if he wasn’t making a mistake in the sum he was asking. To my surprise he replied, "Well, if you’re bargaining I’ll settle for $2,500." I gladly paid him that sum but my conscience bothers me in regard to the $500 I saved as a result of this double error. What is the right thing to do?
Answer: A similar question was put to Rabbi Yitzchak Zilberstein, Rabbi of the Ramat Elchanan community in Bnei Brak, and he compared the settlement made in your case to that of one made as the result of a mistake of a rabbinical judge.
In Shulchan Aruch Choshen Mishpat (25:5) we find a case of one of the litigants in a financial lawsuit consenting to a compromise settlement in order to avoid taking an oath which the judge had imposed on him to substantiate his claim. If the judge subsequently realizes that he had erred in requiring the oath, the compromise that had been reached on its basis is no longer binding since the concession made in reaching the compromise was the result of a mistake.
In the case you mentioned, the painter also was not consciously forgoing payment due him but simply forgot the price you initially agreed upon. If you loan someone $3,000 and later ask him to pay the $2,500 he owes you, would you consider that a forfeiting of your claim to the other $500 that you subsequently remember he owes you as well?
In conclusion, give the painter the $500 you saved as a result of his double error.