At the time of the initial sale of the unit, the initial purchaser shall execute and deliver to the Borough's administrative agent, or an administrative agent appointed by a particular developer, a recapture note obligating the purchaser (as well as the purchaser's heirs, successors and assigns) to repay, upon the first nonexempt sale after the unit's release from the restrictions set forth in 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.