I would imagine that Sam ends up back with the Cs because he, as someone else noted, has the right to stop this deal from happening.
To me, this indicates that it is a prelude to something else or at least allowing the possibility of something else before the deadline.
As was also pointed out, this deal could help SAC more than just getting money. It could enable SAC to complete some other deal that they are working on....which then could trigger the next stage of a deal with BOS.....
while it's always nice to save money, IMO the main reasons to do this deal are to open up the roster spot in order to take back more players than you give out in a deal or Sam going to SAC helps SAC complete a deal and then there will be a favor coming back our way....
maybe it will turn out being what in effect is a three way deal, but with just two stages....
either way, i expect Sam back in BOS because why else would he waive his right to stop the deal?