Mo. Rev. Stat. § 33.130: Setoff in suits brought by state allowed, when.

In all suits brought in behalf of the state, no debt or claim shall be allowed against the state as a setoff, but such as have been exhibited to the commissioner of administration, and by him approved or disapproved, except only in cases where it shall be proved, to the satisfaction of the court, that the defendant, at the time of his trial, is in possession of vouchers which he could not produce to the commissioner of administration, or that he was prevented from exhibiting the claim to the commissioner of administration by absence from the state, sickness or unavoidable accident.