We want to buy a mosque. The receiving end (an old lady) wants the money on a monthly basis, until she dies. Is it permissible to buy the mosque, although we don't really know the precise price (e.g. it could be she'll live 2 years and it could be much more, Wa'allahu 'Alam)? Please provide evidence from Qur'an and Hadith.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is a condition for the validity of the transaction that its value or price is fixed and known. This is better to avoid any future disagreement or problems between the two parties. This is why Islam forbids some kinds of trade transactions because of their ambiguity resulting from some unknown facts in the deal itself. From what we stated above, we conclude that the transaction mentioned in your question is unlawful because it contains an injustice to one party.

In fact, one of the parties will have to suffer some prejudice. The woman might die after a short period before recovering the real price of the land. So, she will be the loser. On the other hand, she might live for a long time and then take more than her right. In this case, it is the seller who suffers unfairness.

Allah knows best.