Is it permissible to renege on a vow to buy a phone for the wife's brother and substitute it with something else with his consent, or to withdraw from the vow entirely?

Undertaking a vow (nadhr) is disliked. If a Muslim vows an act of obedience, he is obligated to fulfill it. If the vow is conditional, it must be fulfilled when the condition is met. It is not permissible to substitute it or retract it. As for vowing a permissible act, scholars have differed regarding its validity. The correct view is that it is not valid, but fulfilling it is recommended to avoid scholarly disagreement.