Thurs Nedarim 53

The Tanna Kamma says that if one makes a Neder from dates, he's permitted to eat date honey.

Ran asks: why is this different from whey that is Assur to r' Yossi if you make a Neder from milk since it's called "milk's whey?" Even the Rabanan only argue because they called it regular whey. However, they would agree here where the product is called date honey.

The Ran brings the Rashba that answers: that whey is different since it doesn't change it's appearance as it was in the milk, so we consider it one and the same. Over here, the honey change its appearance as it was while it was part of the date.

The Ran answers: since there is many types of honey, we need to be specific which honey we refer to, so we call it date honey. Therefore, this doesn't make it its actual name, but a description. However, there is no other whey but what comes from milk. Therefore, there is no reason to call it milk's whey unless it's part of the actual name.