What is the ruling on engaging in murabaha transactions with a usurious bank that does not own the house before selling it to us, considering the absence of Islamic banks, and is it permissible to agree with the seller on an additional amount unknown to the bank to avoid tax, and is this considered riba (usury)?

If you need to deal with a usurious bank in a permissible transaction, there is no harm in it. The mentioned transaction is a permissible Murabaha, and the bank has the right to oblige you to fulfill the promise to purchase or bear any actual damages it incurs due to breaching the promise. As for the "black money," if it is between you and the homeowner without any relation to the bank, there is no objection to it from the perspective of usury. However, if it is for the purpose of evading legally due taxes, then it is not permissible.