In the event that the lessor fails to credit said rental advance to his successor in interest as required by this subsection, the successor in interest shall, without regard to the nature of the transfer, assume liability for crediting of the rental advance, and payment of all interest thereon to the tenant in accordance with the provisions of this section; provided, however, that if the tenant still occupies the dwelling unit for which the rental advance was given, said successor in interest may satisfy such obligation by granting the tenant free use and occupancy of the dwelling unit for a period of time equivalent to the period of time covered by the rental advance. The liability imposed by this subsection shall not apply to a city or town which acquires title to property pursuant to chapter sixty or to a foreclosing mortgagee or a mortgagee in possession which is a financial institution chartered by the commonwealth or by the United States.