Is it permissible to give money to a merchant in exchange for a monthly profit that is not fixed but does not fall below a certain limit, knowing that I am unemployed and have health conditions, and is zakat obligatory on this money?

Zakat is obligatory on the amount if it reaches the nisab by itself or by combining with other funds, and a full year (hawl) has passed on it. The nisab is equivalent to 85 grams of gold or 595 grams of silver. If the money is a loan to the merchant, it is not permissible to take an amount in return for it, as this is considered usury (riba).