Should a minor fulfill his vow?

The vow of a minor is countless because he isn`t required to meet the Sharia rulings at such an age, and it is desirable for him to fulfill that vow once he reaches puberty.

Is it permissible to fish on Saturday?

All Perfect Praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions. 

It is permissible for Muslims to fish on any day, but the sons of Israel are the ones forbidden to fish on Saturday. Moreover, there is no evidence in Islamic Sharia prohibiting Muslims from fishing on that day. And Allah Knows Best.

A man donated a burial plot, but before registering it in favor of the Ministry of Awqaf and before burying anybody there he said that he didn`t want to donate it and planted it. What is the ruling of Sharia on this?

All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

If he had endowed it as a cemetery, then he can`t take it back, even if he didn`t register it in favor of the Ministry of Awqaf. If he said: "I have endowed this plot as a cemetery, then it becomes an endowment." However, if he didn`t endow that plot-but it was his intention to do so-then he is allowed to back down. And Allah The Almighty Knows Best.

What is the ruling on the financial leasing transaction in commercial bank?

Any transation that is involved in the unlawful is impermissible. And Allah Knows Best.