What is the ruling on buying a car from a bank that undertakes to pay its full price, with the buyer paying one-fifth of the amount to the bank, and then the bank deducts monthly installments from the buyer's salary that exceed the remaining four-fifths?

The transaction mentioned, if it involves the bank paying the remaining price of the car on your behalf, and then collecting it from you with interest, this is usury (riba) and is not permissible. However, if the bank buys the car and takes possession of it, then sells it to you at a profit, this is a permissible Murabaha transaction according to Sharia, provided it adheres to the regulations. These regulations include the bank bearing responsibility for the destruction of the good before delivery and the liability for return due to a hidden defect.