Is a contract concluded under the system of lease-to-own considered void, and its proceeds unlawful and usurious, if the contract was signed twenty days before the bank acquired ownership of the property, knowing that the bank had previously agreed with the seller and received an upfront sum from the lessee, which it considered part of the rent?

The lease-to-own contract you entered into is void because the bank leased the house to you before acquiring ownership of it, and this is impermissible based on the general statement of the Prophet, peace and blessings be upon him: "Do not sell what you do not possess." However, due to your ignorance of the Shariah ruling at the time of the contract, and because the contract has become binding and entered into the execution phase, and you have taken possession of the house, this is excused, and you are permitted to benefit from the house. This is because if a Muslim enters into a void contract and takes possession while unaware of its prohibition or acting under an interpretation, then it is affirmed for him, and he is not required to annul it.