At the settlement for an assisted project under a right of first purchase, the purchaser shall execute and record, in a form that the Secretary approves, a covenant running with the land. The covenant shall restrict the use of all assisted units to residential rental property for assisted households, or to some other purpose approved by the Secretary, for at least the greatest of: the remaining term of the mortgage as of the date of its prepayment; the remaining term of the rental assistance agreement, including all stated renewal terms, as of the date of its termination; and 20 years.