The additional payment authorized by this section shall be made only to a displaced person who purchases and occupies a decent, safe, and sanitary replacement dwelling within 1 year after the date on which such person receives final payment from the displacing agency for the acquired dwelling or the date on which the displacing agency’s obligation under
section 4625(c)(3) of this title is met, whichever is later, except that the displacing agency may extend such period for good cause. If such period is extended, the payment under this section shall be based on the costs of relocating the person to a comparable replacement dwelling within 1 year of such date.