Is it permissible to profit from selling in installments, where one partner contributes capital and the other effort? And is it considered usury to give the customer the amount of money for him to buy the commodity, and then for him to repay it in installments with profit?

The partnership of two individuals, one with his capital and the other with his effort, in a permissible endeavor is Islamically permissible, and the profit is divided between them according to their agreement. Installment selling is permissible in principle, even if the total installment price exceeds the cash price.

However, giving the customer a sum of money for him to purchase the commodity himself, and then for him to repay it in installments with an increase, is a prohibited usurious loan. The correct approach is for both of you to genuinely purchase and own the commodity, and then sell it to the customer in installments.