What is the ruling on selling the questioner's share of the apartment to his brother, knowing that the apartment's lease contract ends with ownership after 5 years, and that the price of the share will be paid from a gift originating from usurious bank interest? And is abstaining in this case obligatory or recommended for the seller?

The door of wara' (scrupulousness/piety) is wide, encompassing the abandonment of doubtful matters (shubuhat) and some permissible things as a precaution. For instance, abandoning doubtful matters is obligatory to clear one's conscience, while abandoning some permissible things is out of fear that they might become a means to the forbidden. However, a fatwa (religious edict) is based on the apparent permissible and forbidden, not on wara'.

If forbidden wealth is due to its method of acquisition, not its essence, then it is forbidden only to its acquirer. It is permissible for someone who acquired it from him through a permissible means, such as a gift. This is like accepting a gift from a person who works in a usurious bank.

If a Muslim believes a certain transaction to be permissible and receives money from it, it is permissible for other Muslims to deal with that money, even if they do not believe that transaction to be permissible. This is because upholding a Muslim's ijtihad (independent reasoning) or taqlid (following a qualified scholar) takes precedence.

Therefore, if you own the apartment, it is permissible for you to sell it to your brother, even if he will pay the price with usurious money, while gently advising your mother to stop dealing with usurious banks. The usurious profits that their owners dispose of by giving them to the poor and needy, your brother may accept them if he is poor or needy.