Mo. Rev. Stat. § 144.130

Refunds made by the person during the preceding calendar month or calendar quarter to purchasers, on account of tangible personal property, substances, services and things returned to the persons shall be allowed as a deduction from the gross receipts required to be stated in the returns filed with the director of revenue; provided, the person had theretofore included the said refunded receipts in a return made by such person and had paid the amount imposed by sections 144.010 to 144.510 with respect thereto; provided, the seller has returned to the purchaser any and all tax previously paid by the purchaser at the time of the purchase.