This happened in the 90's with Sean Gilbert. He held-out into the regular season, and the Rams ended up trading him to the Carolina Panthers. Gilbert's only option was to not play, and his only hope of changing his situation was if the Rams traded him.
IMO, the only way Donald walks away without the Rams receiving any compensation is if he does not play during the 2017 and 2018 seasons, and the Rams do not place the Franchise Tag on him in 2019. I don't know what, if any, pay Donald might receive during that time.
