What is the ruling on taking an unstable sum of money from an in-law who works as a money changer and is given money to pay employees' salaries, and who determines the percentage of profit himself?

The questioner does not mention any work performed by his relative that would be suitable for Mudarabah (definition) (profit-sharing) with another person's money, nor is there clarity regarding the connection between the money changer's job and investing money. Profit from money given to another is not permissible unless it is through Mudarabah or participation in a permissible trade, such that he has an agreed-upon share of the profit. Profit merely from giving money is usury (riba), especially with a guarantee of the principal or the profit, for this is a loan that draws benefit, and that is usury.