My wife does not work, but she receives an income from her inheritance in addition to the returns of her share in the inheritance. She vowed to slaughter an animal sacrifice, but she does not have enough cash to buy it. Can I give her the rest of the money to fulfill her vow and slaughter the animal sacrifice?
May Allah reward you.

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 is His Slave and Messenger.

There is nothing wrong with a husband helping his wife by giving to her money so that she can buy an animal sacrifice and fulfill her vow. This is considered a form of helping a Muslim to carry out a righteous deed, which is recommended in Islam. The Prophet, sallallaahu ‘alayhi wa sallam, said: "Allaah, the Exalted, will continue to  aid His slave as long as he continues to aid his (Muslim) brother." [Muslim]

Moreover, Jaabir reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "He who amongst you is capable of doing good to his brother, let him do it." [Muslim]

There is nothing wrong with the wife accepting her husband's money in order to complete the money needed to buy her vowed animal sacrifice (and fulfill her vow).

Allaah Knows best.