I have the answer, but it's an arcane (and interesting) piece of baseballia. Basically, yes, Danny can be optioned the next two years if the Nationals want. He just has to pass through optional waivers, in which another team could technically claim him (but never do because the Nats could just take him back and probably teams don't out of courtesy to each other given roster limitations).
Optional Waivers:
Optional major-league waivers are required when optioning a player who is more than three calendar years removed from his first appearance on a major-league roster. This procedure allows a club to send a player to the minor leagues while keeping him on the 40-man roster. Because optional waivers are revocable, players usually clear in this scenario. In the unlikely event a player is claimed, his club may not option him to the minor leagues, and any subsequent waiver request during the same period becomes irrevocable. Never has a player been claimed on optional waivers (which are obviously different from trade waivers and outright waivers).