Missouri courts have held that a completed inter vivos gift cannot be revoked by the donor once the gift is delivered and accepted by the donee.[1]  However, if the donor makes a gift subject to a condition, the donee’s failure or refusal to perform the condition or violation of the condition constitutes grounds for revocation of the gift by the donor.  The donor retains the right to revoke the gift unless or until the condition is satisfied.  The state courts have held that a gift given in contemplation of marriage is made upon the implied condition that the gift will become absolute when the marriage takes place.  Therefore, a gift given in contemplation of marriage, though absolute in form, is a conditional gift and may be revoked by the donor if the marriage engagement is breached by the donee.

[1] Clippard v. Pfefferkorn, 168 S.W.3d 616 (Mo. Ct. App. 2005)