donation

Promising to donate money from one's salary

Q: I would like to ask regarding the following matter:

I promised Allah to donate a specific amount from my first salary. I am doing an internship and I am a student. Now I am deciding to give that amount in Bakra Eid to buy a cow. Is it okay? Because my family might also eat some meat from that cow but almost 85% is distributed to the needy. My intentions were to give that money to some specific individuals who cam benefit from that money. But now I am thinking to contribute my amount to buy a cow in Bakra Eid. Can you please suggest what should I do?