What is the ruling on purchasing an apartment with an increasing installment system, knowing that a non-refundable down payment is made?

If the total price of the apartment is known to both parties at the time of the contract, and they do not stipulate an increase in price upon delay, then there is no harm in that. The difference in price between an immediate sale and a deferred sale is permissible in Islamic law. However, if the price is unknown and unspecified at the time of the contract, then the sale is invalid because it is a condition for the validity of a sale to know the quantity of the sold item and its price.