What is the ruling on dealing with Islamic banks if the contract is based on profit and loss, and does the ruling differ if the agreement is based solely on variable profit?

Dealing with Islamic banks: If the contract is Mudarabah, the capital must not be guaranteed because it is subject to profit and loss. Guaranteeing it transforms it into a forbidden loan. However, if the contract is Murabahah to the buyer, it is not permissible to increase the debt in exchange for extending the period after the debt has been settled. The criterion for what is permissible and forbidden is not the abundance of return, but rather the command of Allah Almighty.