Does the difference in the total tuition fees paid to a private university, based on the method of payment (direct monthly payment or a percentage of salary after graduation), fall under the category of "two sales in one sale"?

It is permissible to pay tuition fees in installments after graduation if this has been agreed upon. There is no harm if the deferred price is higher than the cash price, provided that both parties definitively agree on one of the two options (cash or installments). It is not permissible to increase the agreed-upon fees upon late payment, as this is forbidden interest (riba). It is also forbidden to contract without specifying the method of payment, such that the matter remains ambiguous between immediate or deferred payment, as this falls under the category of "two sales in one sale," which is prohibited by Islamic law. The total amount of installments or the monthly installment must be specified and known. It is not permissible for it to be unknown, such as a percentage of an unknown salary.