Is it permissible to fast on behalf of the dead who died owing missed fast to make up?
His relative should fast on his behalf, and it is permissible for the guardians to give permission to non-relatives of the dead to fast on his behalf as well. And Allah Knows Best.
Is it permissible for a man to marry up to four wives (polygamy in Islam) for pleasure although he can`t afford that?
It is desirable for the man to have one wife if there was no need for him to have more, if he can't afford marriage forthcoming expenses.
Is it permissible for a person who had vowed to give a certain amount of money to another, but didn`t find the latter to donate that money to the mosque?
In principle, the vowing person should abide by his vow as much as possible. Therefore, if he couldn`t find the person that he had made the vow for, then the vow itself is countless and nothing is due on its maker. However, if the latter happens to find the former later on then, he has to give him that money.
Is it permissible for a young man to marry a young woman who is only seven months older than him?
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.
From an Islamic perspective, it is permissible for a man to marry an older woman. And Allah The Almighty Knows Best..