A gift in the form of an engagement ring would not be given unless for the promise of a future marriage. There is a statute in the California Civil Code 1590 that addresses this type of gift.
According to the statute, the circumstances surrounding the breakup determine the proper owner of the engagement ring. The "giver" may recover the ring or the value of the ring if the "receiver" refuses to enter into marriage. The "giver" also has a right to recover the ring if the contemplated marriage is abandoned by mutual consent.
Although not explicitly stated in the statute, subsequent case law has determined that the "receiver" of the engagement ring is entitled to keep the ring if the "giver" refuses to enter the marriage without any fault of the "receiver." Therefore, if the "giver" is the one who backs out of the wedding, then the "giver" does not have the right to get the engagement ring back.