Poirier + one of Edwards, Williams, Williams, Ojeleye, or Wanamaker gets it done salary-wise.
They probably want draft compensation too. I'd be willing to give a protected first that converts to two seconds for Bertans. He's a good player on a sizable contract that could be retraded at some point down the line.
As another aside, if they traded two-for-one, it would open up a roster spot for Evan Turner, who is a good teammate, veteran presence, and gives us another ball-handler.
As another aside, if Ainge wanted to trade Smart for a big, he could without giving up depth if they signed Evan Turner.
For example:
1. Poirier and Ojeleye (and a protected first) for Bertans
2. Sign Evan Turner
3. Smart and Edwards to the Wolves, Covington and Memphis pick to Pacers, Turner to the Celtics
Celtics Roster
Turner-Theis-Kanter
Tatum-Bertans-Williams-Williams
Brown-Langford-Green
Hayward-Turner
Walker-Wanamaker-Waters