Counteroffer after offer is sent

When offer is sent to counteragent, there are two buttons: "Accept" and "Decline", there is no way for counteragent to accept the offer with a different sum without contacting initiator directly.
There should be a third button "Counteroffer" which asks counteragent of a new sum and then sends it to initiator with the same three options.
I'm not sure how it would be integrated into the current initiator-counteragent data model, because they switch roles in this scenario. Maybe it could be implemented as a special case. In any case, this needs more thought.