Exhaust venting into air return?
This is an acceptable solution for multi-tenant buildings where the exhaust fans from multiple units vent in to a common duct. The booster fan in the floor vent makes sure that the air pressure in the room being vented is higher than the common duct. The booster fan can pull air through the central AC/hear air handler even if it's not running. This lightens the load on the exhaust fan.