Bava Batra 79 - Selling a Donkey and its Foal

One who sells a donkey has sold its foal, but if one sold a cow, he has not sold its calf. If one sold a dump, he has sold its manure. If one sold a pit, he has sold its waters. If one sold a beehive, he has sold the bees. If one sold a dovecote, he has sold the doves. In all these cases, the contents are considered subsidiary to and are sold along with the primary item that contains them.

Why is a donkey different from a cow? In this case, he said, "I am selling you a nursing donkey." For the cow, this may be interpreted as buying the cow for its milk, but the non-kosher milk of a donkey is useless to the buyer, and therefore the "nursing" adds its foal to the sale.