Commentary for Bava Metzia 4:6
עַד מָתַי מֻתָּר לְהַחֲזִיר, בַּכְּרַכִּים, עַד כְּדֵי שֶׁיַּרְאֶה לְשֻׁלְחָנִי, וּבַכְּפָרִים, עַד עַרְבֵי שַׁבָּתוֹת. אִם הָיָה מַכִּירָהּ, אֲפִלּוּ לְאַחַר שְׁנֵים עָשָׂר חֹדֶשׁ מְקַבְּלָהּ הֵימֶנּוּ, וְאֵין לוֹ עָלָיו אֶלָּא תַרְעֹמֶת. וְנוֹתְנָהּ לְמַעֲשֵׂר שֵׁנִי וְאֵינוֹ חוֹשֵׁשׁ, שֶׁאֵינוֹ אֶלָּא נֶפֶשׁ רָעָה:
Until when is it permitted to retract (for coin ona'ah)? In cities, [where there are money changers], until he shows it to the money changer; in villages, [where there are no money changers], until Sabbath eve, [when he comes to spend it for (provisions for) the Sabbath meal, when he will discover if it will be accepted in payment.] If he recognized it, even after twelve months. [This is what is meant: If he (the giver of the coin) were a chasid, and wished to go beyond the letter of the law — if he recognized it as the sela he gave him, he should accept it even after twelve months.] And he has only resentment against him. [This is what is meant: If he is not a chasid and does not wish to accept it from him, he (the receiver) has only resentment against him, the receiver having caused himself the loss by not having returned it in time.] And he gives it for (redemption of) second-tithe, without apprehension, for he is a mean person. [This refers to a coin rubbed out to the point of ona'ah. He gives it for (redemption of) second-tithe and need have no apprehension of asimon (see 4:1), second-tithe being redeemed only with a coin (stamped with a design), for it is called a "coin." For one who does not take it at its worth as a coin, but only as cast silver, is regarded as a mean person.]
Bartenura on Mishnah Bava Metzia
English Explanation of Mishnah Bava Metzia
If he [that had given the coin] recognized it, even after twelve months he should accept it [in return], but [if he does not accept it the one who received it] only has a cause for complaint [against him].
And it is permitted to use it for redeeming the Second Tithe without concern, for [he who would not receive it] is but only a cheapskate.
If a person should receive a fraudulent coin (one that weighs less than it is supposed to), he may demand that the person who gave him the coin replace it with a better coin. However, there is a time limit for making such a demand. In large towns where money-changers who could check the coin would be found, he only has as much time as it would take to bring it to a money-changer. In small towns he has until the eve of the Sabbath, since it can be assumed that most people from small towns will visit the market and see money-changers before the Sabbath.
If the person who gave him the coin recognized the coin he should exchange it even after the time mentioned above has long lapsed. However, if he refuses to exchange the coin the person who received it has no legal claim against him. The most he has is the right to make a formal complaint, which may damage the reputation of the person who gave the coin but will not legally compel him to exchange the coin.
Finally the mishnah teaches that one may redeem his Second Tithe with this coin, since anyone who would not accept the coin is only being cheap. The Second Tithe was the second ten percent of a person’s produce. It had to be brought to Jerusalem and eaten there. Since it was difficult to carry all of the produce itself to Jerusalem a person was allowed to redeem the produce with money, and then bring the money to Jerusalem to use it to buy food.