As the title mention, i have 3 classes : User, Offer, Message. There is a constraint : A user can't send a message to another user if he's not interessted in his offer. How do i model this case?! where i should add the proper attributes?!
This is a constraint which belongs to a use case.
There are too many different ways this constraint can be implemented. So you can not infer that constraint to any class construct directly.