Is it permissible to spend money entrusted as an أمانة (trust) to treat an ill father without his knowledge, given the unavailability of personal funds to cover the treatment costs?

It is not permissible to dispose of entrusted money without the owner's permission. If the father gives permission, it is permissible to spend from his money on his treatment if it is in his best interest. If he is of sound mind and refuses treatment, it is not permissible to dispose of his money without his permission. Scholars have differed on the ruling regarding medical treatment (tadaawi), and the preponderant opinion is that it is permissible to forgo it unless the illness leads to death and the medication is certainly beneficial. In such a case, treatment becomes obligatory, based on the Almighty's saying: "And do not throw [yourselves] with your [own] hands into destruction" and "And do not kill yourselves." Since cancer is a serious disease and there is no certainty of recovery from it, and if the medication is not likely to succeed, then treatment is not obligatory. Therefore, it is advisable to consult doctors and try to gently persuade the father if the success of the medication is confirmed. It is also recommended to seek treatment through Ruqyah Shar'iyyah (Islamic incantations) and prophetic medicines.