[Retraction from an undertaking]

The answer – and Allah is the One who grants success – is that the children may retract what they undertook for their mother so long as she is alive; and likewise in any undertaking. If a man says to another: “Marry [a woman], and whatever becomes obligatory upon you is on me,” then he has the right to retract from this undertaking so long as that man has not married. Once the man has married, it is not valid for the one who undertook (the obligation) to retract from what he undertook.
We only said that they may retract during their mother’s lifetime by analogy with the executor (waṣī); for there is no disagreement that he may withdraw (from the appointment) before the testator’s death, not after his death. Likewise the guarantor may withdraw from the guarantee before the right is established, not after it. And the agent (wakīl) may resign from the agency, and the seller may withdraw from the sale before acceptance; likewise the donor and the one giving charity may withdraw before acceptance – or before taking possession in the case of charity.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2