Is it permissible to pay for a dress, given as a gift to one's mother, from money designated for spending in the cause of Allah, knowing that there was a prior promise to pay for the dress after its completion?

Charity is not obligatory merely by intention. Therefore, you are not obliged to give this money in charity for the purpose you intended, and you may buy a gift for your mother with it. If this gift is accompanied by the intention of drawing closer to Allah by honoring your mother and bringing joy to her, a great reward is hoped for.