The rights of property owners must be considered when their properties are subjected to zakat. The best items are not to be taken as zakat unless the owners freely permit it. Likewise, the rights of the poor should be considered.
A defective animal should not be taken as zakat unless all of the other animals are defective. In such a case, zakat is due to the average of that property. Some proofs for this view are:
1. In the letter of Abu Bakr: “Neither an old or a defective animal nor a billy goat may be taken as zakat.” This also means that zakat cannot be taken in excess of what has been required; unless he wants to pay zakat more then he needs to, then it is permissible.
2. Sufyan ibn ‘Abdullah ath-Thaqafi reported: “Umar forbade the zakat collector to levy zakat on the following: barren ewes (al-‘akulah), a sheep kept at home for milk (ar-rahy), a pregnant ewe (al-rnakhid), or a ram used for breeding (fahl al-ghanam).”
3. ‘Abdullah ibn Mu’awiyyah al-Ghadiri reported that the Prophet, upon whom be peace, said: “Whoever performs these three acts will have had (savored) a taste of belief (‘irnan): He who worships Allah alone, and [believes] that there is no god but Him; he who good-heartedly offers the zakat on his property which will repay him every year; and he who does not offer a very old sheep, a mangy sheep, a sick sheep, a mean and low sheep, or a ewe which produces only a small amount of milk. You should offer one from the average. Verily, Allah asks you to offer neither the best nor the worst.” [It was related by Abu Dawud and at-Tabarani with a good transmission].
Taken with slight modifications from Fiqh-us-Sunnah by As-Sayyid Sabiq