How is the zakat calculated for a sum of money that started at fifty thousand in August, then gradually increased to eighty-five thousand in July, and is managed for the purpose of trade with a monthly profit?

The aforementioned transaction is a void mudarabah (profit-sharing partnership) because the profit is fixed. It must be annulled. The owner of the capital is entitled to his principal and its profit, and the agent is entitled to a fair wage (ujrat al-mithl). The money in your relative's possession is considered a trust (wadi'ah). This trust is subject to zakat after it is received, or the owner can pay its zakat annually from what he possesses. If you receive the money from your relative, you pay its zakat when a year has passed on its principal, and the profit is zakat-eligible with it. If the mudarabah capital consists of trade goods, their value is assessed at the time zakat becomes due, and their hawl (zakat year) is based on the hawl of the price for which they were purchased.