Maaser Sheni, Chapter 4, Mishnah 5. This mishnah continues to deal with ways of avoiding the obligation to add a fifth when redeeming Maaser Sheni. The case before us: a person is standing at the threshing floor where the grain and the Maaser Sheni are being processed, and he has no money with him. Since he has no money, he cannot hand coins to his friend standing there and tell him, "Redeem the produce with this money."
The solution the mishnah offers:
He says to his friend: this produce is given to you as a gift. Once the produce belongs to his friend, he can then say: let this produce be deconsecrated and become ordinary produce - that is, let it cease to be Maaser Sheni - upon the money I have at home. In the words of the mishnah quoted above: "Ein b'yadi kesef, aval yesh li kesef babayis" - I have no money with me, but I have money at home. In other words, with the money lying in his house he redeems his friend's produce - the very produce he has just given him as a gift. And of course, he relies on his friend and knows that he will return the produce to him afterward.
A difficulty: Maaser Sheni may not be given as a gift:
As we already mentioned in the previous mishnah, a person may not give Maaser Sheni as a gift. Therefore the mishnah must be explained in one of two ways:
The mishnah is speaking of a situation in which the produce is still tevel, before Maaser Sheni has been separated from it, and it is the friend who will separate the Maaser Sheni.
Alternatively, this mishnah follows the approach of Rabbi Yehudah, that Maaser Sheni is not the property of Heaven and does not belong to Hashem, but rather it is the owner's property and belongs to him - and therefore it may be given as a gift.
The Gemara's precise reading:
The Gemara points out that the case must be one in which the friend, too, has no money at home. For if the friend did have money at home, there would be no need at all to transfer the produce to him as a gift; he would simply say to him, "Do me a favor and redeem this Maaser Sheni upon the money you have at home." From here we see that the friend has no money at home. One can also say that even if he does have money, he may not want to transfer the sanctity of Maaser Sheni onto his coins, since he would then have to bring that money up to Jerusalem.
In summary: The mishnah teaches that when a person has no money with him at the threshing floor, he gives the produce to his friend as a gift, and afterward he himself redeems his friend's Maaser Sheni produce upon the money lying in his house - and in this way he is exempt from adding the fifth.