763. It may be stipulated that a gift, either of present property or in contemplation of death, made in a contract of marriage, shall be suspended, revocable, reducible, or subject to changeable or indeterminate reservations and rights of resumption, although the effect of the stipulation depends upon the will of the donor. If, in the case of reservations and of a right of resumption, the donor do not exercise his or her right, the donee retains the full benefit of the gift to the exclusion of the heir of the donor.