Is zakat obligatory on money deposited monthly with a brother for the purpose of buying a house from him, and how is it calculated if it is obligatory?

Purchasing a house from a brother takes one of the following forms:

1. Saving Money: The brother saves money as a trust. When the amount is complete, the sale takes place. In this case, zakat is obligatory on the saver if the money reaches the nisab and a year has passed. 2. Loan: The brother takes the money and benefits from it as a loan. This transaction is forbidden if the loan is a condition for the sale, due to the saying of the Prophet (ﷺ): "It is not permissible to combine a loan and a sale." If it is not a condition, then there is no harm, and zakat is obligatory on this debt if it is owed by a solvent person. 3. Installment Sale: The sale takes place now with an installment price, and the final contract is delayed until the full price is paid. An initial contract should be written to document the rights, and there is no zakat on the amounts you pay as they have exited your possession.