Numbers 5:9

Every terumah-gift, etc. R’ Yishmael said, “Is terumah-gift brought to the kohein? Does he not seek it, himself, in the granary? Then why does the Torah state, ‘ . . . which they bring to the kohein?’ These are the bikurim-gifts, about which it is stated, ‘[. . . the first fruits of your land] must be brought to the House of Adonoy, your God.’ But one does not know what must be done with them. The Torah therefore states, ‘ . . . to the kohein, it shall belong to him.’ Scripture teaches, concerning the bikurim-gifts, that they must be given to the kohein.

And every separation of all consecrated things of the children of Israel which they bring to the priest shall be his.

AND EVERY HEAVE-OFFERING OF ALL THE HOLY THINGS OF THE CHILDREN OF ISRAEL, WHICH THEY BRING UNTO THE PRIEST, SHALL BE HIS. Scripture is saying that the heave-offering which the children of Israel hallow, that is to say, which they set aside [of the produce] and declare holy, shall be the priest’s, for since there is no specific measure [required by law of the Torah when setting aside] the heave-offering, therefore Scripture said that however much they set aside of the produce [as the heave-offering] shall belong to the priest when the owners bring it and give it to him, but the priest may not take it by force. This is the plain meaning of the verse. And so the Rabbis have said in the Sifre: 5. The “Sifre” is the Tannaitic Midrash on the Books of Numbers and Deuteronomy. It is equivalent to the Mechilta on Exodus, and the Sifra or Torath Kohanim on Leviticus. “Rabbi Akiba says: ‘Scripture comes [here] to teach you that if the owner wishes to declare his whole granary heave-offering, he may do so, provided that he leaves over a small amount’ [as ordinary food].”
We have furthermore been taught there in the Sifre: 5. The “Sifre” is the Tannaitic Midrash on the Books of Numbers and Deuteronomy. It is equivalent to the Mechilta on Exodus, and the Sifra or Torath Kohanim on Leviticus. “Which ‘yakrivu’ (they bring) unto the priest. Said Rabbi Yishmael: But do they bring the heave-offering to the priest? What then is the meaning of the expression, which ‘yakrivu’ (they bring) unto the priest, shall be his? [It is as follows:] Since He stated, The choicest first-fruits of thy land thou shalt bring unto the House of the Eternal thy G-d, but we have not been informed yet what shall be done with them, therefore Scripture states [here], which ‘yakrivu’ (they bring) unto the priest, shall be his — Scripture comes and teaches you that the first-fruits are to be given to the priest.” Now Rashi [in citing here this text of the Sifre] explained [the question] “But do they bring the heave-offering to the priest?” in the following way: “Does not the priest have to go around the granaries asking for it?” But this is not clear. (they ‘bring’) unto the priest, shall be his. The heave-offering, according to Rashi, is to be “brought” to the priest. In the opinion of Ramban, only the priest’s taking it by force is forbidden, while his going around for it, is not prohibited in this verse, …

וכל תרומה , the first tithe the Israelites set aside from their crops for the priests, as well as the ten per cent the Levites set aside for the priests from the tithe they had received from the ordinary Israelites as spelled out in Numbers chapter 18.

וכל תרומה, according to Sifrey 5 the t’rumah of which the Torah speaks here are the first fruit which have to be offered to G’d, as it is customary for the owner of fields or orchards to present these first fruit of each year’s produce (seven species) and to recite the appropriate benediction when presenting same in the Temple. (compare Deuteronomy 26,3). Here the Torah tells us that just as property stolen from a proselyte who died intestate, which was described as “being returned to G’d,” winds up in the hands of the priest, i.e. G’d’s agent, so the bikkurim, the farmer’s gift to G’d has also been assigned by G’d to the priest of the respective roster on His behalf.