At the time of the first sale of the unit, the initial purchaser shall execute and deliver to the administrative agent a recapture note obligating the purchaser (as well as the purchaser's heirs, successors and assigns) to repay, upon the first non-exempt sale after the unit's release from the requirements of this section, an amount equal to the difference between the unit's nonrestricted fair market value and its restricted price and the recapture note shall be secured by a recapture lien evidenced by a duly recorded mortgage on the unit.