I have a many-to-many relationship between 2 objects - contracts & headings.  The junction object is called Contract Headings and is the detail side of the 2 master-detail relationships.

On the headings object, I have a lookup formula to pull back a category number from a fourth object Heading Category.  I've also put a cross-object formula on the junction object Contract Headings to pull this contract number over from the Heading object.  I realize you cannot use cross-object formula fields in roll-up summaries so to get around this, I created another formula field on the junction object to pull over the value from the cross-object formula field (on junction object) for category. 

So to summarize, on the child object in this attempted roll-up, I have a cross-object formula which I cannot use in a roll-up and another formula to give the value in the un-usable cross-object formula...confusing I know.  :)

However, when I try to create the roll-up summary field on the contract object to give the MIN category number from the junction object, the field does not show up. 

I really need a roll up summary field on the contract object to give the MIN of the category number on the heading object.  Any idea how I can make this work?