Is it possible to retract a vow to donate half the dowry if the hoped-for marriage does not materialize, while paying expiation, knowing that the vow-maker was ignorant of the ruling on vows and their prohibition at the time of making the vow?

The questioner must fulfill her vow when her marriage to the man she desires comes to fruition. She is not permitted to retract the vow, because she made an absolute vow and did not specify a time frame or condition that would invalidate it before the fulfillment of what she tied her vow to. Her ignorance of the ruling on vows and their prohibition does not prevent the vow from being valid nor does it negate the obligation to fulfill it.