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, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
We have explained in detail in previous Fatwas the ruling on selling cats mentioning that the opinion adopted by the majority of scholars is that it is permissible. We also stated that the preponderant opinion is that it is forbidden based on the strong evidence that supports it. However, there is nothing wrong if someone follows the opinion that adopts the permissibility if it is based on strong proof or complies with the opinion of trustworthy scholars without following one's desires or pursuing the concessions granted by the scholars. Nevertheless, it is better for a person who views the preponderance of the permissibility to stay away from the divergence of the scholars' opinions in this concern. There are many ways through which one can earn his living and praise be to Allaah The Almighty. Moreover, the Prophet, sallallaahu ‘alayhi wa sallam said, "…so whoever guards against doubtful matters will protect his religion and honor." [Al-Bukhari and Muslim]
As regards the ruling that you ask about according to those who view the permissibility of selling cats, Zakah is due on the money earned thereby if it is done for trade purposes because it is assessed money and this is not problematic. Shaykh ‘Abdullaah ibn Jibreen said: "We have previously stated that trade goods include all items proposed for selling whether big or small, immovable or movable. All that is prepared for trade with the aim of gaining profit should be assessed when a full lunar year passes on it and one-fortieth of it should be given out as Zakah."
Hence, it becomes clear to you that cats intended for trade are included in the term 'trade goods' according to those who view that selling cats is permissible. So, the owner should assess the cats at the beginning of the lunar year and, if its value reaches the Nisaab even by adding it to other forms of wealth in which Zakah is payable, then he has to pay its Zakah (of 2.5%) as long as its value does not become less than the Nisaab during the lunar year.