From: Rabbi Moshe Sofer
To: Rabbi Yuda
The Chasam Sofer rules that donors who pledged a three-year annual sum to a destitute widow may retract or redirect their third-year pledge without formal hatarah, since the widow has since remarried a wealthy man and the children are now fully supported. Based on the umdena that the donors never intended to give under changed circumstances, and following the Rosh's ruling as codified in Shulchan Aruch, the pledge is considered automatically lapsed; nonetheless, he recommends formal annulment as a pious stringency out of deference to a minority view.
Source: Shu"t Chasam Sofer YD §237