What is the ruling on the money I received from my maternal uncle after his death, given that there was an agreement with him, in the presence of witnesses, for the capital to double every two years, but his family gave me less than what was agreed upon, claiming that he had made such a will before his death?

The aforementioned transaction is a void Mudarabah (profit-sharing partnership) because one of the conditions for a valid Mudarabah is that the share of the amil (worker/manager) and the rabb al-mal (financier) must be an undivided portion of the profit (as a percentage), not a fixed sum or a percentage of the capital. If either party stipulates a known sum or a percentage of the capital, the Mudarabah becomes void. In the case of a void Mudarabah, the correct approach is to return the capital to its owner, and the profit should be distributed between the two partners according to the common prevailing division known in such trade, with this percentage estimated by seeking the assistance of experts and trustworthy individuals. In this specific situation, if it is difficult to ascertain the profits after the partner's death, it is best to accept what the heirs offer and to repent to Allah for entering into a forbidden transaction.