All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Pledging yourself to do something is not considered a binding oath or vow. Hence, you are not obliged to offer that money in charity. It is permissible for you to pay that money as expiation, for which you are liable.
However, if you mean that you had made your pledge or covenant using the formula of a vow or what indicates it such as saying, 'It is my duty to offer such-and-such in charity to the people of Syria', or ‘I [hereby] make a covenant with Allaah to offer such-and-such in charity...’ or similar statements, then this is considered a binding vow and it is incumbent on you to fulfill it. The Prophet, sallallaahu ‘alayhi wa sallam, said: "He who vows to obey Allaah, should obey Him..." [Al-Bukhari and others]
In this case, it is impermissible for you to give that money as expiation for your past vow and fulfillment of the vow at the same time (combining the two intentions and paying one sum of money to clear one's liability from both obligations). Instead, you are obligated to pay that expiation independently from the vow for which you are liable.
For further information, please, refer to Fatwa 251359.
Allaah Knows best.