I guess I just don't see the point of your post. Dunn's trade value isn't enough to move him, and Willingham is too old to be worth keeping at the rate this team is going.
EDIT - I should clarify what I meant about Dunn's trade value. It's not enough to move him for what Rizzo is apparently asking.
My point is that Dunn should be traded before Willingham. We don't really know what Rizzo's asking price is. It's impossible to say whether he should or shouldn't pull the trigger in a vacuum, but I'll say this: If you were to put me in the G.M. chair, I'd probably go with a Dunn trade right now, even if the return seems low. The reason being that Dunn is probably at peak value right now.
Next year, he won't fetch as much in a trade because the Nationals will be pressed to deal him given that he'll be in a contract year. Any possible added value to Dunn will be offset by the fact that he'll be in a contract year; thus, I think Dunn is at peak value right now. Given that, and given the fact that I think you would want to keep Willingham, I think trading Dunn is something that needs to be done, not necessarily for the sake of being done, but for the sake of getting more value than you will in the alternative situations.