What should I do, as I vowed to slaughter an animal upon buying a car, but when I bought it, I became financially constrained and did not slaughter it?

If the intention behind the slaughter is to give the meat as charity, then the vow must be fulfilled upon the realization of the condition it was made contingent upon and the feasibility of fulfilling it, because it has become a debt and a trust. This is based on the Almighty's saying: "And let them fulfill their vows," and the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah the Almighty, let him obey Him."

However, if you are unable to fulfill it due to an inability that is not expected to cease, then you must offer an expiation for an oath (kaffarat yameen). This is based on the Prophet's (peace be upon him) saying: "Whoever makes a vow he cannot fulfill, its expiation is the expiation of an oath, and whoever makes a vow he can fulfill, let him fulfill it," and "The expiation for a vow is the expiation of an oath."