Is it permissible for a Muslim who vowed to allocate a portion of his salary to support a widow with an orphan to marry a widow with an orphan to fulfill his vow, knowing that he has no issue marrying a previously married woman of good character, and that the vow was for the purpose of obligation and avoiding dereliction? And if he marries her, will she cease to be considered a widow, and will he then be obligated to search for another widow to give the allocated portion to?

What is considered binding in a vow (nadhr) is its verbalization, not merely the intention. If it is verbalized, it becomes binding, and its fulfillment becomes obligatory upon the occurrence of the matter upon which it was conditioned. If the one giving charity intended to donate a portion of his salary to a widow or an orphan's mother, then marrying her does not count as fulfilling the vow. This is because the wording of the vow was clear: it was about paying a portion of the salary to the widow, not marrying her. Furthermore, spending on her is originally an obligation in exchange for enjoyment (of marriage), and charity necessitates transfer of ownership. Moreover, by marrying her, she loses the status of a widow. Therefore, the vow must be fulfilled exactly as it was vowed, and Allah has praised the believers who fulfill their vows. This type of conditional vow is disliked, and if the one who made the vow is unable to fulfill it, then expiation (kaffarah) is required.