Mo. Rev. Stat. § 86.177
The pension reserve fund shall be the fund in which shall be held the reserves on all pensions granted to members not entitled to prior service credit or to their beneficiaries and from which such pensions and benefits in lieu thereof shall be paid. Should a beneficiary retired on account of disability be restored to active service and again become a member of the retirement system his pension reserve shall be transferred from the pension reserve fund to the pension accumulation fund. Should the pension of a disability beneficiary be reduced as a result of an increase in his earning capacity, the amount of the annual reduction in his pension shall be paid annually into the pension accumulation fund during the period of such reduction.
§ 86.177, RSMo