I'm also more interested in potential trade targets shaking free than the UFA market, at least this year. Green or Carlsson doesn't sound like a good idea to me as Letang replacements.
Carlsson is going to cost Letang money - and that's even if he makes it to FA. With the cap going up Washington is (and has said) that they going to try and sign him. Whether you agree or not that Carlsson is a comparable to Letang is another matter... but he won't be much (if any) cheaper. And I said that at the start of the season well before he was tied for 1st in points among blueliners.
Green is basically another Schultz. Not quite the same, but much more like him then Letang. We could probably make that work if our 3rd pairing was rock solid... but then you're probably talking about spending huge dollars on the blueline to make that happen. Something like this:
Dumoulin - Green (at 5m or so)
Oleksiak - Schultz
Maatta - XXXXX (another Green/Schultz or better type of player, at another 5m or so)
That's not impossible... but we'd probably be much better off keeping Letang and using Ruhwedel and spending the money saved on a forward.