What is the legal minimum weight (nisab) for a calf vowed to be slaughtered on Eid al-Adha after a certain matter is achieved?

If you vow to slaughter an animal with the intention of drawing closer to Allah (Qurbah), like an Udhiyah (definition) (sacrifice) or charity, then your vow is binding and must be fulfilled. However, if you did not intend Qurbah, then the vow is not binding.

For a cattle sacrifice, it is required that the animal be at least two years old. A vow transforms a recommended act into an obligatory one.

If you specifically designated the animal, for example by saying, "I vowed to slaughter this calf," then you must slaughter it, even if it does not meet the criteria for a valid Udhiyah.