What is the Sharīʿah ruling on increasing the price of the service of connecting homes to the potable water network when paying in installments, and is it permissible to pay in installments with the ability to pay in one lump sum?

The aforementioned increase has two possibilities: Either it is agreed upon in the contract that the buyer chooses installments, and thus the contract is made for a specific amount to be paid in installments over defined periods, and this is permissible. Or, the contract is made with the buyer for both the immediate and deferred prices, and then he chooses later; this is forbidden because it is a transaction of two sales in one, which the Messenger of Allah, may Allah bless him and grant him peace, prohibited.