Should a young man donate his first salary as alms to fulfill his covenant with Allah, or should he return it to his father from whom he borrowed money that is doubtfully unlawful?

If the vow is made in the form of a solemn pledge (nawther), such as saying, "It is a duty upon me for the sake of Allah to give the first salary in charity," then it is obligatory to fulfill it. This is based on the Almighty's saying: "And let them fulfill their vows" (22:29) and "And fulfill the covenant of Allah when you have covenanted" (16:91). Allah Almighty condemned those who break their promises, as in His saying: "And among them are those who made a covenant with Allah, [saying], 'If He should give us from His bounty, we will surely spend in charity...'" (9:75) The Prophet, peace be upon him, said: "Whoever vows to obey Allah, let him obey Him."

However, if the pledge was merely an internal thought without being uttered aloud, then nothing is incumbent upon you, and you may dispose of the salary as you wish.

Your intention to repay your father's money that he spent on you is not obligatory if the father was donating it willingly or if it was an obligatory expenditure.

The fact that your father's money has a suspicion of being unlawful does not make all of it unlawful, and it is permissible to deal with him concerning it.