What is the ruling on a usufruct (manfa'ah) that was gifted to the poor and reached them through purchase from a rich person? And what is the ruling on accepting an invitation from a relative who works in a usurious bank, and accepting his gifts?

If the intent is that a benefit was bestowed upon specific poor people, but some individuals prevented certain poor people from their right to it by purchasing it with their money, then the benefit is lawful for them, and the sin is upon those who withheld their right and took compensation for it. If the intent is otherwise, it must be clarified.