Is it permissible for a wife to take money from her husband instead of selling her gold, which she vowed to donate as charity, and would that be considered fulfilling her vow?

If the intention is coupled with the wording of a vow (nadhr), then it becomes obligatory to fulfill it. However, if it is merely an intention in the heart, then nothing is obligatory. In the case that it is a vow, the vowed item itself (a specific piece of jewelry) must be given, and it is not permissible to substitute it with its monetary value.