This is true - but the Phillies, if they choose, can trade Buchholz along with a large bucket of cash, which means their market for trading Buchholz can be much larger than Boston's market was - because Boston needed to shed the salary. The Phils can just eat it.
Buchholz at $13.5 million may not command much. Buchholz at $3.5 million would be a bargain, worth trading something of value for. Somewhere between those two numbers, there could be a deal - Buchholz + cash in return for prospect of real value.
I doubt if the Phils are looking for such a deal (or any Buchholz deal, really) at this point. The strategy appears to be to have him pitch; if he pitches effectively, he becomes that much more attractive to the market come mid-July. The Phils don't need a prospect in return this spring; they need prospects for the long run, and July's a fine time to make a deal.