Mo. Rev. Stat. § 408.210
The payment of four hundred dollars or less in money, credit, goods, or things in action, as consideration for any sale or assignment of, or order for the payment of wages, salary, commissions, or other compensation for services, whether earned or to be earned, shall for the purpose of this chapter and any act regulating loans or punishing usury be deemed a loan secured by such assignment, and the amount by which such assigned compensation exceeds the amount of such consideration actually paid shall for the purposes of this chapter and any act regulating loans or punishing usury be deemed interest or charges upon such loan from the date of such payment to the date such compensation is payable, and such transaction shall be governed by and subject to the provisions of this chapter, provided that nothing in this section contained shall be construed as authorizing the assignment of wages, salaries, commissions, or other compensation for services not earned at the time when such purported sale or assignment thereof is made. In case any transaction defined in this section results in a greater rate or amount of interest or charges than is permitted by law, all contracts taken in connection with such transaction shall be void and the lender shall have no right to collect, receive, or retain any principal, interest or charges.
§ 408.210, RSMo