What is the ruling on donating half of the amount vowed for the couple after the wife's financial situation has stabilized?

Whoever vows an act of obedience must fulfill their vow. If you intended to assist your brother with a specific amount of money on an ongoing basis, you must fulfill that. However, if you vowed to assist him until he became self-sufficient or for a specific period, and you fulfilled that, then there is nothing wrong with you reducing or stopping the amount. If your brother's wife is generous and spends on him and herself, then he is not required to continue the assistance. But if she demands maintenance or considers what he pays a debt, then he must continue his vow.