To Gain and to Give: Hilchot Tzedakah, Day 6
Halacha considers a pledge to contribute money to tzedakah as a vow, and as such one is obligated to keep their pledge. For example, one who responded affirmatively to a solicitation phone call is required to keep their pledge, even if they subsequently had a change of heart. Even one who did not articulate a commitment but merely mentally resolved to give tzedakah should follow through with their good intention (to be continued).
רמ"א ס' רנח סי"ג