Maybe they think they can flip him at the deadline for a better prospect than what they gave up?
several articles I read speculating on Canha as a potential FA this offseason said the option was a reasonable price in the abstract but that MOAC (Milwaukee's owners are cheap). I thought that were he to hit the market, a 2 year deal in the $8 million per year range might have made sense for the Nats. He could help the transition in the OF and 1st/dh while guys develop.