Is it permissible to substitute a vow by slaughtering a calf whose value is equal to a number of sheep?

The basic principle concerning vows (nadhr) is to fulfill them as they were made. Substitution is not permissible unless it is with something better, and this is the view of the Shafi'i and Hanbali schools. As for the Hanafis and Malikis, they hold that the vowed act must be fulfilled exactly as it was vowed, and deviation is not permissible except in cases of inability.

Cattle (cows/oxen) are considered better than sheep for sacrifice and drawing closer to Allah, based on the Prophet's (peace be upon him) saying: "Whoever goes (to the mosque on Friday) in the first hour, it is as if he sacrificed a camel; and whoever goes in the second hour, it is as if he sacrificed a cow; and whoever goes in the third hour, it is as if he sacrificed a horned ram." This is also because cattle are more expensive, have more meat, and are of greater benefit.

Therefore, it is obligatory to fulfill the vow of slaughtering a calf, as it is better than slaughtering a number of sheep. It is also safer and more cautious to adhere to the specified vow and not deviate from it.