I'm not sure. IIRC, I had read (probably on these boards itself), that the salary cap for next year is projected to be around 95M. Per HoopsHype, Spurs are projected to be at 93M next year, per the current roster.
Of course, a lot can change, but if the above numbers are correct, then they might not be able to just sign a player, and might have to do it via trade.
This is entirely theoretical of course. I don't know the numbers (was basing it on 95M, and HoopsHype data, but might be wrong), don't know if the Spurs are even interested in Ben, and if something is feasible. Was also just an example.
That said, does someone know more about the projected cap next year?