Ma'aser Sheni, chapter five, mishnah 9. This mishnah teaches how a person fulfills the mitzvah of bi'ur (removal) when he and his produce are not in the same place.
If the time of bi'ur arrives and he has produce from which he has not yet separated Terumah, the tithes, and the other gifts he is obligated to give, he must "designate them by name." Usually this expression means that he must declare over the produce that this portion is Terumah and that portion is ma'aser, but here that is not the intent: here it means that he must make sure that the kohen or the Levi has actually received the gifts and is actually holding them.
The incident with Rabban Gamliel and the elders on the ship:
The mishnah brings an incident that illustrates this: Rabban Gamliel and the other Tannaim of that period were traveling on a ship, and the time of bi'ur arrived. These were the declarations:
"Isur she'ani asid lamod nasun liYehoshua, u'mekomo muschar lo" - Rabban Gamliel said about his produce: the ten percent of ma'aser that I am destined to measure out and give to the Levi is hereby given to Rabbi Yehoshua ben Chananyah, who was a Levi. And the place where the ma'aser is lying is rented to him - and since the ma'aser is lying in a domain rented to Rabbi Yehoshua the Levi, he acquires it and the ma'aser becomes his.
"Isur acher she'ani asid lamod nasun laAkiva ben Yosef sheyizkeh bo la'aniyim, u'mekomo muschar lo" - an additional ten percent that he had to measure out in that year. And when did this take place? After the third year and after the sixth year, which are the years in which bi'ur is performed, and those years are not years of ma'aser sheni but years of ma'aser ani, which must be given to the poor. Therefore Rabban Gamliel said that this ten percent should be given to Rabbi Akiva ben Yosef, so that he would acquire it on behalf of the poor - for he was a charity collector and was responsible for the funds of the poor. Here too, the place where the produce is lying is rented to him, so that he would have a domain belonging to him, and consequently he would acquire it on behalf of the poor by virtue of its lying in his domain.
"Isur sheni she'ani asid lamod nasun leElazar ben Azaryah, u'mekomo muschar lo" - once this was done, Rabbi Yehoshua, who was a Levi and now had in his domain that ten percent of ma'aser that had originally belonged to Rabban Gamliel, said: ten percent of the ten percent in my hand - that is, one hundredth - is hereby given to Rabbi Elazar ben Azaryah, who was also there and who was a kohen. This is the terumas ma'aser we are discussing, and he gives it to Rabbi Elazar ben Azaryah, and the place, the area in which it is lying, is rented to him.
"V'kiblu zeh mizeh sachar": each of them accepted payment from the other for the rental, in order to effect the acquisition, so that each one was actually the owner of the area he needed to hold in order to acquire the tithes or the terumas ma'aser.
In summary: in this mishnah we learned that one whose produce is far from him at the time of bi'ur must designate it by name, and a declaration alone is not enough - one must make sure that the kohen and the Levi are actually holding the gifts. The incident with Rabban Gamliel and the elders on the ship demonstrates this: Rabban Gamliel gave the ma'aser to Rabbi Yehoshua the Levi and the ma'aser ani to Rabbi Akiva, the charity collector, to acquire it on behalf of the poor; Rabbi Yehoshua gave the terumas ma'aser to Rabbi Elazar ben Azaryah the kohen - and each one rented the place of the produce to the recipient and accepted payment, so that the gifts would be lying in the domain of the recipient and he would actually acquire them.