Screen Capture on Retina MacBook Pro at a Lower Resolution?
I just ran into this exact issue the other day. Try RetinaCapture: it's a free app that captures screenshots in both 1x and 2x simultaneously (or, alternatively, just one or the other). And it's a "true" 1x capture: it redraws at 1x rather than taking the 2x version and scaling it down.

It also supports the same types of captures as the built-in Grab.app – full screen, timed screen, selection, or window – and you can save either to the clipboard or a file.