The plan shall provide that the low income units upon resale may be sold to low income buyers, and the moderate income units to either low or moderate income buyers; provided, however, that the administering agency may establish reasonable provisions for waivers of this condition on a case by case basis in the event it finds that a particular unit may not feasibly be sold subject to this condition. In the event that the administering agency grants such a waiver, it shall provide that the unit be sold at the formula price and that the resale controls remain in effect for any subsequent sale of the unit.