Responsa for Nedarim 179:9

Q - A vowed to give one pound to charity, upon the occasion of his son's circumcision. Although the occasion had not, as yet, taken place he now seeks absolution by a scholar from the vow.
A - He can not be absolved from the vow until the occasion of his son's circumcision actually takes place. Then, if he so desires, he can seek absolution by a scholar from the vow.
This responsum was addressed to R. Samuel