Mo. Rev. Stat. § 87.310
When any member terminates his or her employment as a firefighter and withdraws his or her accumulated contributions from the retirement system, he or she ceases to be a member of the retirement system. If he or she is reinstated as a firefighter he or she will again become a member of the retirement system as a new member with no creditable service prior to his or her termination. However, any member currently employed as a firefighter may repay the retirement system his or her total accumulated contribution withdrawn at the time of his or her termination and an amount of interest on such contribution at the same rate per annum as allowed in the member's savings account in the same period. Such repayment shall occur within two years after August 13, 1984, and when made the member shall then receive full credit for service prior to the date of his or her termination.
§ 87.310, RSMo