When does the zakat year begin for money resulting from the sale of property, and is this money added to the funds on which zakat is paid annually from salary, or is it subject to zakat separately?

The house you bought with the intention of renting it out is not subject to zakat on its value. Rather, zakat is due on its income (rent) if it reaches the nisab and a year has passed over it. Offering the house for sale does not change its ruling as long as the intention of selling is not for profit. If the house is sold, zakat becomes obligatory on its price after a year has passed over it. It is permissible to combine the money received from selling the house with your salary and pay zakat on both together as an early payment, or to pay zakat on each when its hawl is due.