It depends.

Young guys that just got promoted to the 40 man roster after being in AA or AAA ball sign "mixed" contracts which pay them their MLB salary only if they are in the majors, and a minor league salary if they are in the minors.

But once a guy signs a long term contract, or has been around a few years and goes to arbitration and earns 7 figures, that contract is usually guaranteed. As are major league free agent contracts. So if one of THOSE guys goes down to the minors (for real -- an injury rehab assignment in the minors doesn't count; they are still on the ML disabled lit) -- they would get paid the full amount. Of course that is rare, once a guy has established himself and makes 7 figures, he probalby isn't going back to the minors. But it did happen this week with Nationals' starter John Lannan, who signed a $5M/year multi-year contract with them last year, but got beat out for the last starter role and sent to AAA. He is earning $5 million while playing in Syracuse.