Machatzit HaShekel and Ma'aser - Yechaveh Daat 1:87

Can a person use his regular ma'aser funds to give machatzit ha-shekel? Rav Yosef rules that one cannot do so. He bases this initially on a teshuva of the Maharam MiRutenberg (#74), who says that once the money is designated for one cause, it cannot be transferred to be used for another one. Rav Yosef then discusses whether ma'aser kesafim is a mitzva or simply a minhag, but concludes that even if it is a minhag, if a person commits himself to giving it, it takes on the status of a neder and thus it becomes obligatory. As such, he follows the view of the Maharil (#56) who says that ma'aser money cannot be used for matanot la-evyonim, and he extends that to include machatzit ha-shekel. However, he notes that if a person has fulfilled his obligation in giving matanot la-evyonim to two poor people and he still wants to give more, he can give that additional amount from ma'aser funds.