Is it permissible to use part of the money allocated for charity—which is paid regularly but not to a specific recipient—to buy the sacrificial animal for Eid al-Adha?

The Udhiyah (sacrificial animal) is a confirmed Sunnah and better than giving its price in charity. Therefore, there is no harm in spending a portion of one's salary to purchase the Udhiyah; in fact, that is preferable.