i found this on wikipedia, which answers my question:
consider the practical reality of purchasing a vehicle from an Islamic bank under an allegedly "zero interest" loan. The procedure, generally, is that the client tells the Islamic bank which vehicle he or she would like to own. The Islamic bank then purchases that vehicle in its name, and sells it to the client at a marked-up price, under an agreement that the new marked-up price of the vehicle must be paid in a certain number of installments of a certain time period. Thus a $20,000 car might cost $35,000 if purchased from an Islamic bank at "zero interest," 5 year loan. One may reason that the Islamic bank charges the extra $15,000 on top of the $20,000 cost of the car because money has a time value (that is to say, a payment of $20,000 5 years from now is worth less than a payment of $20,000 today). This is also why a $20,000 car could cost $35,000 if the purchase were financed by an interest bearing loan issued by a non-Islamic financial institution. However, where under an Islamic bank one would end up paying the extra $15,000 no matter how quickly he pays his 'loan', under non-Islamic banking one can end up paying less if he repays his loans quicker.
i found this on wikipedia, which answers my question:
consider the practical reality of purchasing a vehicle from an Islamic bank under an allegedly "zero interest" loan. The procedure, generally, is that the client tells the Islamic bank which vehicle he or she would like to own. The Islamic bank then purchases that vehicle in its name, and sells it to the client at a marked-up price, under an agreement that the new marked-up price of the vehicle must be paid in a certain number of installments of a certain time period. Thus a $20,000 car might cost $35,000 if purchased from an Islamic bank at "zero interest," 5 year loan. One may reason that the Islamic bank charges the extra $15,000 on top of the $20,000 cost of the car because money has a time value (that is to say, a payment of $20,000 5 years from now is worth less than a payment of $20,000 today). This is also why a $20,000 car could cost $35,000 if the purchase were financed by an interest bearing loan issued by a non-Islamic financial institution. However, where under an Islamic bank one would end up paying the extra $15,000 no matter how quickly he pays his 'loan', under non-Islamic banking one can end up paying less if he repays his loans quicker.