To Gain and to Give: Hilchot Tzedakah, Day 7
It was previously mentioned that one who pledged to give tzedakah is obligated to keep their pledge. What does one do if they pledged a specific amount of money and then forgot how much they pledged? In such an instance one must give to tzedakah the amount of money that was possibly originally pledged. Thus, one who is unsure if they pledged $36 or $54, but knows with certainty that they did not pledge more than that, gives $54 to tzedakah.
שו"ע ס' רנח ס"ג