פירוש על קידושין 115:15

The mishnah does not refer to an Israelite who gives terumah that he had to separate to a woman. Rather, it refers to an Israelite who inherited tevel, untithed produce, from his mother’s father. There is terumah inside this tevel, and this tanna considers the terumah to be a separate entity. The Israelite owns the terumah and can do with it what he wants, although he may not eat it. The woman will now be able to sell it. That is why he can perform kiddushin with it.