Is it permissible to increase the amount agreed upon in a contract for designing a dwelling after two years of project completion, given that the full agreed-upon amount has not been received, due to rising prices?

It is not permissible to increase the agreed-upon amount due to rising prices, because fixed debts in a certain currency are repaid with their equivalent, not with their value. The consideration for the fulfillment of fixed debts is the equivalent, not the value, and it is not permissible to link fixed debts to price levels.