Ma'aser Sheni, Chapter 3, Mishnah 1. The Mishnah teaches: "Lo yomar adam la'chaveiro: Ha'al es hapeiros ha'eilu li'Yerushalayim lachaleik" - a person may not tell his friend to bring up the Ma'aser Sheni produce with him to Jerusalem in order to divide it there between them, so that one takes a portion and the other takes a portion.
The reason for the prohibition:
When phrased this way, it emerges that the portion given to his friend is payment for his trouble in bringing the produce up to Jerusalem, and this is like someone selling Ma'aser Sheni produce or paying off a debt with it. And Ma'aser Sheni may not be sold, nor may a debt be repaid with it.
The permitted wording:
"Ela omer lo: Ha'aleim shenochlem venishtem bi'Yerushalayim" - in this language he may approach his friend. The difference between the two formulations:
"Lachaleik": the division itself is the whole point - this one takes his share and that one takes his share, so the produce comes as a form of payment.
"Shenochlem venishtem": even though his friend takes a portion of the produce, there is no division here but rather a shared meal, like two people sitting down to eat together.
"Aval nosnin zeh lazeh matnas chinam":
This principle was already stated in the first Mishnah of the tractate: Ma'aser Sheni may not be sold, but people may give it to one another as a free gift. And not simply a gift - since an ordinary gift has the status of a sale - but a free gift, such as when they eat the produce together.
This closing line is brought as the reason for the ruling stated in the body of the Mishnah: one may tell his friend "Bring them up so that we can eat them in Jerusalem," because this is the very same free gift mentioned in the first Mishnah of the tractate - a shared meal that is not even a gift resembling a sale, but a complete and absolute gift, and therefore it is permitted.
In summary: a person does not tell his friend to bring the Ma'aser Sheni produce up to Jerusalem with him "to divide it," since the portion he gives him is payment for his trouble, and this is like selling Ma'aser Sheni or repaying a debt with it. But he may tell him "Bring them up so that we can eat and drink them in Jerusalem," and even though his friend takes a portion, this is not a sale and not even a gift resembling a sale, but a completely free gift - which is permitted with Ma'aser Sheni.