I made a vow to Allah, the Exalted, that I would slaughter a calf if a certain thing happens. Can I offer the monetary value of the calf instead of slaughtering it when my wish is fulfilled?

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.

Fulfilling vows is an emphasized obligation. So, you have to fulfill your vow according to what you have intended it to be if the thing upon which you based your vow happens. It is impermissible for you to offer a substitute to what you had vowed as long as you can afford it exactly as you intended it to be. The Prophet, sallallaahu ‘alayhi wa sallam, said: "If anyone makes a vow that he is unable to fulfill, its expiation is the same as that for a broken oath, but if anyone makes a vow that he is able to fulfill, he must do so." [Abu Daawood, An-Nasaa’i, At-Tirmithi and Ibn Maajah] Moreover, the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever vowed to obey Allaah, must obey Him; and whoever vowed to disobey Allaah, must not disobey Him." [Al-Bukhari and others]

Allaah Knows best.