Do you really really really want Harper beyond this season? He's wildly inconsistent, which wouldn't bother me so much, but he's Bryce Harper, savior of the universe. When he's going bad, the team often follows, as if he's what they should follow. Hell, he even has HYPE in his name. Players should be led astray by the poor performance of a teammate, so they, as professionals, should be above it. A manager should also help prevent it. But he's Bryce Harper. A talented enigma...who's team has never NOT underachieved. Let him be someone else's grey-hair inducer.
With the Nats lineup of prospects (the best being outfielders) I'd let Harper leave. I'd attempt to buy out Zimmerman, too. Start with what's left and see if the window is really closed.