notice period

Just wondering when the tenant gives a 21-day notice to the real estate and the tenant decides to move out say after 14 days, does the tenant still have to pay the rent for the full 21 days? How does this work?

We recently bought a property as a first home buyer. There were tenants living in the property and we decided to keep them there for a few months. We are currently renting in another place. The owner of the unit we are renting has decided to sell the unit. The real estate called me today...