What is the ruling on selling a computer program for an immediate payment and another mandatory monthly payment as a maintenance contract, which entails stopping the program in case of non-payment, knowing that the seller does not inform the client about the possibility of changing the monthly amount unless asked?

The condition of maintaining the sold item at the time of the sale contract is permissible if it does not contradict the contract. This falls under the category of "sale with a condition." The duration of the maintenance and its fee must be specified at the time of the contract, because ambiguity in either of these will invalidate the lease contract. You do not have the right to change the fee during the period except with the lessee's consent. You must commit to providing the benefit in exchange for the agreed-upon fee. However, it is not permissible to stop the program if the buyer refuses to pay the fee.