(a) For the purpose of determining the amount of unfunded vested benefits allocable to an employer for a partial or complete withdrawal from a plan which occurs after September 25, 1980, and for the purpose of determining whether there has been a partial withdrawal after such date, the amount of contributions, and the number of contribution base units, of such employer properly allocable—

(1) to work performed under a collective bargaining agreement for which there was a permanent cessation of the obligation to contribute before September 26, 1980, or

(2) to work performed at a facility at which all covered operations permanently ceased before September 26, 1980, or for which there was a permanent cessation of the obligation to contribute before that date,

shall not be taken into account.

(b) A plan may, in a manner not inconsistent with regulations, which shall be prescribed by the corporation, adjust the amount of unfunded vested benefits allocable to other employers under a plan maintained by an employer described in subsection (a).