Is it permissible to take money previously donated by a benefactor for pharmacy studies after transferring to the Faculty of Arts without informing her, and is it permissible to consider it a debt owed to her, to be repaid after graduation?

The grant money is subject to the condition stipulated by the granting entity, based on the Prophet's saying, "Muslims are bound by their conditions." If the donor did not stipulate your continued enrollment in the Faculty of Pharmacy, then reference should be made to custom and the predominant assumption. If it is predominantly assumed that she is assisting you in the Faculty of Pharmacy due to high tuition fees or a specific purpose related to your pharmacy studies, then it is not permissible for you to take the money without her permission. Otherwise, there is no harm in taking it, because the predominant assumption takes the place of certain knowledge in most rulings of Sharia. It is best to inquire from the donor about her condition.