Gittin 85 - Get with Unusual Conditions
If one gives his wife a Get and says, "You are hereby permitted to any man except to my father or your father," or "to a slave or an idolater," the divorce is valid, since her marriage to any of these man would not be binding.
But what if the husband makes a condition that she can not marry a particular minor? Now the minor can not marry, but eventually he will become adult. By comparison with a minor girl who can divorce because eventually she will marry as adult, the Get above is declared not complete separation, thus void.