What is the ruling on making an exception in a monetary vow (Nudhur)?

Primary text

If a person vows, 'It is incumbent upon me for God that I give charity of one hundred dirhams, if God wills' (Lillahi 'alayya an atassadaqa bi-mi'ati dirham, in sha'a Allah), nothing is binding upon him. This is because the vow (Nudhur) is classified as an expiable oath, thus allowing the exception based on the general principle concerning oaths.