What is the ruling on paying off the remaining house price with an usurious increase, knowing that I cannot pay the amount in one lump sum?

If you know that the loan will turn into an interest-bearing (riba-based) loan after five years, then your agreement to the contract is unlawful because it condones usury. The assumption of repaying the loan within the permitted period does not change the ruling. What is obligatory upon you is to repent and to repay the principal amount of the loan without the interest, except in a case of necessity.