Michigan Compiled Laws, Chapter 38. Civil Service and Retirement § 38.1379a

Sec. 79a. (1) Subject to subsection (2), if the compensation amount used for computing payment for a purchase of service credit under section 64, 69, 69c, 69d, 69e, 71, 74, 74a, 74b, 75, 77, or 78  1 before the effective date of the amendatory act that added this section exceeded the full-time or equated full-time compensation earned in the school fiscal year used in that computation, the payment for that purchase shall be recomputed using the greater of either the full-time or equated full-time compensation amount earned for the school fiscal year in which payment was made, or the highest school fiscal year compensation previously earned by the member. A refund shall be made based upon the recomputation.

(2) A refund pursuant to this section shall be made only to a member, deferred member, or retirant who makes written application to the retirement board in a form determined by the board.