Maaser Sheni, Chapter 4, Mishnah 4. This mishnah deals with the idea of a legal device (ha'aramah) regarding maaser sheni - the way in which one can sidestep the halachah that obligates the owner to add a fifth when redeeming his maaser sheni.
The basis for permitting this device:
The Yerushalmi explains that the basis for permitting this way around the halachah is the wording the Torah uses regarding maaser sheni: "Ki yevarechecha Hashem Elokecha" - "for Hashem your God will bless you." Since a blessing is mentioned in connection with it, the Sages learned that it is proper to add to the owner's benefit in this matter, and therefore he was permitted to find this way around the obligation so that he comes out ahead. For the same reason the halachah was stated above in Mishnah 2 - two mishnayos earlier - that one who redeems may redeem at the cheaper wholesale price, that is, at the price at which the retailer buys and not at the price at which he sells.
The wording of the mishnah:
"Ma'arim al maaser sheni" - one may sidestep the obligation of adding a fifth to maaser sheni. How? "Omer adam livno ulevito hagedolim, ule'avdo ulshifchaso ha'ivrim: Heilach ma'os elu ufdei lecha maaser sheni zeh" - a person says to his adult son or daughter, or to his Hebrew servant or maidservant: here is this money, and with it redeem this maaser sheni.
Why specifically these people? Because these are the people a person relies upon, that they will not run off with the money but will do with it as he wishes. He gives them the money as their own, and now they redeem the maaser sheni as outsiders do, and they are not required to add a fifth - since this is not their own maaser sheni.
In contrast: "Aval lo yomar ken livno ulevito haketanim, ule'avdo ulshifchaso haKena'anim, mipnei sheyadam keyado" - but he may not say this to his minor son or daughter, or to his Canaanite servant or maidservant, because their hand is like his hand. These two categories of people do not work for this device:
His minor son and daughter - there is no possibility of using this method through them.
His Canaanite servant and maidservant - non-Jewish slaves, through whom this also does not work.
The reason: the halachah is that their hand is like his hand. When they acquire something, they do not acquire it independently; rather, the acquisition ultimately belongs to the master of the house. It emerges that placing the money in their hands accomplishes nothing, since they are merely an extension of him, and therefore he would still be obligated to pay the fifth - and the device would not help.
In summary: in this mishnah we learned about the permitted device regarding maaser sheni, whose basis is the wording "Ki yevarechecha Hashem Elokecha" - since a blessing is mentioned in connection with it, it is fitting to act for the owner's benefit (and from here too comes the permission in Mishnah 2 to redeem at the wholesale price). The device is carried out by giving money to his adult son or daughter, or to his Hebrew servant or maidservant, who then redeem the maaser sheni as outsiders and without adding a fifth; but it does not work with his minor son and daughter or with his Canaanite servant and maidservant, whose hand is like his hand and whose acquisition is not an independent acquisition.