What is the ruling on paying installments for a home purchase with interest for 20 years after the law changed, given that I was unaware of it?

If the issue is the higher price of installment payments compared to the cash price, then there is no harm in that, provided the total amount is the agreed-upon price of the residence with the owner for installment payments over 20 years, as the increased payment period is met with a share of the price.