[Ruling on retracting forbearance (musāmaḥah)]

Answer – and Allah is the One who grants success – is that this is not valid, even if the dropping of the debt occurred with the wording of “gift” (hibah). That is because gifting a debt is a form of relinquishment, and from here they said that gifting a debt does not need acceptance.
What is valid to retract from is conditioned upon certain conditions, among them: that the retraction be from a specific item (ʿayn), not from a debt – except where the gift of the debt was in exchange for a consideration and that consideration was not received; then he has the right of retraction.
Another condition is that the specific item still exists and has not been consumed.

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