What is the ruling on buying dollars in cash and then selling them at the (higher) check rate, and repeating the process to benefit from the price difference?

This transaction falls under the ruling of sarf (currency exchange), and it requires immediate possession (taqabud) if the currencies are of different types, and both equality (tamathul) and immediate possession if they are of the same type. A bank check is considered as possession if it is certified and has an immediate, withdrawable balance. However, if it is post-dated or has no immediate balance, it is not considered possession. It is permissible to sell a check for another currency, such as dollars, provided there is immediate possession at the time of the contract. This requires the check to be certified and to have an immediate balance.