Does studying become unlawful and earning a salary forbidden if you made a covenant with Allah not to commit a specific sin, then you committed it, and said: "O Allah, if I commit this sin again, make my studies forbidden for me"?

Declaring what is lawful as unlawful, such as saying, "It is unlawful for me if I do such and such," is considered an oath. If the one who swore the oath violates it, he must offer expiation for breaking an oath, because something does not become unlawful merely by one's statement. It is stated in "Matalib Uli Al-Nuha" that whoever declares something lawful as unlawful, other than his wife, must offer expiation for breaking an oath if he does that thing, based on the Almighty's saying: "O Prophet, why do you prohibit what Allah has made lawful for you?" and His saying: "Allah has already ordained for you [Muslims] the dissolution of your oaths." So, if he says, "If I commit the sin, I will make studying unlawful for myself," he must offer expiation for breaking an oath. As for saying, "By Allah, I will not do it," this is another oath, but only one expiation is required for multiple oaths concerning the same matter.

However, if the statement is, "If I commit the sin, O Allah, make studying unlawful for me," this is a supplication and not an oath, and nothing is required for it. The oath is binding with the statement, "By Allah, I will not do this again." The expiation for an oath is to feed ten needy persons, or to clothe them, or to free a slave. Whoever cannot find any of these must fast for three days, as stated in the Almighty's saying: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended in your oaths. The expiation for this is feeding ten needy people from the average of that which you feed your [own] families or clothing them or freeing a slave. But whoever finds not [the means] then a fast of three days. That is the expiation for your oaths when you have sworn. And guard your oaths."