Sure but I think if the Spurs were going to lock up DDR, then they would have already done so.
There's the point that they would have to build an extension on top of his $27.7 million dollar salary and that's why they're letting him hit FA, so they can re-sign him to a more reasonable deal.
Then again, I assume DeMar wants similar money over 3 years (something like 3 years, $75 million). Trading for Collins and then re-signing Collins and DDR is possible but that's going to be a very expensive team with a log jam on the perimeter, with DDR reverting back to the 3, and the team would still having Murray, White, Vassell and KJ.