Is Zakat on money obligatory for the money remaining after selling produce and paying Zakat on crops? And is Zakat on money obligatory for a debt I loaned to my brother, which I do not currently possess? And if the debt cannot be repaid for years, is Zakat obligatory on it every year or upon its possession?

If the questioner pays Zakat on crops, then he is not obliged to pay Zakat on the price of the crops unless it reaches the نصاب (minimum threshold) and a hawl (one lunar year) has passed. As for the debt owed by the brother, if he is solvent and not procrastinating, then Zakat on the debt is due along with the questioner's funds. But if he is insolvent or procrastinating, the questioner pays Zakat on it for one year when he receives it.