What does the Torah mean when it writes "v'Ish Es Kodashav lo Yihyu"?
Rashi #1: 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'eumos and Ma'aser. 1
Rashi #2: It implies that if someone does not give Ma'aseros, 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 them as Kodshim.
What is the latter half of the Pasuk coming to teach us?
Rashi: 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 - the right to give it to the Kohen of his choice.
Rashbam: It teaches us that if someone designates something that is eligible for Matanos Kehunah - such as Charamim, he must give it to the Kohen.
Seforno: It teaches us that, once the owner gives the Kohen his Matanos Kehunah, it belongs to the Kohen, and neither he (the original owner) nor another Kohen, can take it away from him.
Ramban: As specified in Korach (Perek 18).