Mo. Rev. Stat. § 474.425

Property which a testator gave in his lifetime to a person is treated as a satisfaction of a devise to that person in whole or in part only if the will provides for deduction of the lifetime gift, or the testator declares in a contemporaneous writing that the gift is to be deducted from the devise or in satisfaction of the devise, or the devisee acknowledges in writing that the gift is in satisfaction. For purpose of partial satisfaction, property given during the testator's lifetime is valued as of the time the devisee came into possession or enjoyment of the property or as of the time of death of the testator, whichever occurs first.