Can't grant landed title to my vassal with a du jure duchy claim on the county

One of my vassals is the High Chiefdom of Temes, and he wants to own Bacs, a county in his Du Jure Duchy. I have the title of Chiefdom of Bacs, and I want to give it to him. When I try, the option is grayed out, with the explanation I must have a landed title which he can hold.

Well I do have that title, and he wants it, so I am confused as to why it can not be granted. I can however grant it to another vassale.

Does anyone know what I can do to grant it to him, or why I currently can not?