Sforno on Leviticus 22:16/ספורנו על ויקרא כ״ב:ט״ז

1והשיאו אותם עון אשמה, they themselves, when they declared the items in question (for instance t’rumah) as sacred cause themselves to become guilty in the event they would trespass and make profane use of them. The Torah does not allow you to apply the principle הפה שאסר הוא הפה שהתיר, that just as one may declare something forbidden, the same person can also declare it as allowed. (D’mai,6,11) The reason why we cannot apply this principle in this case is אני ה' מקדשו, seeing that G’d Himself elevated this item to a status of holiness, a mere mortal, even the donor, cannot remove such sanctity at his own discretion.