I think the timing of it actually goes like this (this is a hypo - there's no guarantee washington would offer detroit more than dallas):
Detroit negotiates with Dallas and Washington for compensation. Washington offers more. Detroit agrees in principle with Washington and allows Roy to negotiate with Washington, but not Dallas. Roy negotiates with Washington and a deal is likely. If Roy doesn't like what Washington is offering, then Detroit says "tough" you sign with them or play here another year.
Detroit wouldn't let him negotiate with both Washington and Dallas out of the fear that Dallas would offer Roy more money, making Roy not want to be traded to Washington - bad for Detroit because the Skins offered better draft picks.
Teams will only let players negotiate with other teams if the trade compensation has been agreed upon. In this scenario Roy would have no idea what Dallas would be willing to pay him - that's why a deal with Washington is likely, because he has no way to play teams against each other.