What is the ruling on selling a commodity by installment after purchasing it based on a customer's request with a profit margin ranging between 25-30%? And what is the way to invest small capital in light of the decreasing value of currency? And is it permissible to save by buying gold and selling it after its price increases?

It is permissible for your father to purchase tools and sell them in installments for more than he paid for them, provided that the sale meets the stipulated conditions. This is because a person is allowed to sell what he owns for a price agreed upon with the buyer. The way to invest a small amount of money is to invest it in what Allah has permitted, such as buying and selling, and to stay away from forbidden things, for Allah "destroys interest and gives increase for charities." It is also permissible to buy gold and save it to sell when its value increases, with the obligation to pay zakat on it when a year has passed and it reaches the nisab (85 grams).