What does the Torah mean when it writes "ve'Ish es Kodashav lo Yih'yu"?
Rashi #1 and Seforno: It means that the Kodshim that the owner is obligated to give to the Kohen and to the Levi also belong to him, inasmuch as he has Tovas Hana'ah - the right to choose the Kohen and Levi to whom he wants to give his T'rumos and Ma'asros. 1
Rashi #2: It implies that if someone does not give Ma'asros, his field will ultimately produce only one tenth of what it used to produce, so that he ends up with the Ma'aser that he failed to give.
Ramban (citing Targum Onkelos) and Targum Yonasan: It means that a man's Kodshim that the Torah does not obligate the owner to give to the Kohen, 2 belong to him and he is permitted to eat them, in spite of the fact that the Torah refers to the as Kodshim.
And what is the latter half of the Pasuk coming to teach us?
Rashi and Rashbam #2: It is coming to teach us that someone who gives the Matanos that he is obligated to give, will end up with a lot of money.
Ramban: It teaches us that even the Kodshim that the Torah obligates the owner to give to the Kohen, 1 he retains Tovas Hana'ah
Ramban: As specified in Korach (Perek 18).