What are the consequences for someone who has made multiple vows, some of which he does not remember, and some of which are difficult to fulfill immediately or require a large sum of money, given that he is a student?

Vows of obedience are obligatory to fulfill, as Allah Almighty says: "And let them fulfill their vows," and as the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." What you do not remember, strive to recall and fulfill it. It is permissible to delay what you cannot currently do until you are able to. If you are permanently unable to fulfill it, then its expiation is the expiation for an oath (feeding ten poor persons, or clothing them, or freeing a slave, or fasting three days if you cannot find any of these).