An employee vowed to slaughter (a cow) when he reaches the retirement age, but when he reached the retirement age, he wouldn’t be able to fulfill this vow.  

Is it permissible for him to slaughter a sheep or two instead of a large sacrifice(cow), or what should he do?

    All Praise due to Allah, and peace and blessings of Allah be upon the Messenger, his family, companions, and people who follow him: -

The vow must be fulfilled as an act of obedience and righteousness. This is according to the Hadith of the prophet (pbuh) said: "He who vows to obey Allah, should obey Him. But he who vows to disobey Allah, should not disobey Him."

But If a person can’t fulfill the vow, the expiation (for such a vow) is the expiation for breaking an oath.

On the authority of Ibn Abbas, may Allah be pleased with them [a discontinued Hadith]: "Whoever makes a vow and is not able to fulfill it, the expiation for that is the expiation for breaking an oath. Whoever makes a vow and is able to fulfill it, let him do so."

Therefore, the questioner must fulfill his vow if he is able, but if he is unable to fulfill his vow, then he must expiate for an oath which is "feeding ten needy people or clothing them". But whoever cannot find [or afford it], he must fast three days. Regarding the slaughter of sheep is not sufficient for this vow.