I am an amateur at this stuff. But here's what the CBA that governs Carson's contract says about it. In Appendix C where they provide a sample player contract, this is the relevant section:
EXTENSION. Unless this contract specifically provides otherwise, if Player becomes a member of the Armed Forces of the United States or any other country, or retires from professional football as an active player, or otherwise fails or refuses to perform his services under this contract, then this contract will be tolled between the date of Player’s induction into the Armed Forces, or his retirement, or his failure or refusal to perform, and the later date of his return to professional football. During the period this contract is tolled, Player will not be entitled to any compensation or benefits. On Player’s return to professional football, the term of this contract will be extended for a period of time equal to the number of seasons (to the nearest multiple of one) remaining at the time the contract was tolled. The right of renewal, if any, contained in this contract will remain in effect until the end of any such extended term.
I underlined the important part. Basically, it means that when a player retires with time remaining on his contract, it is as if the contract is frozen and would just start back up where he left off should the player decide to return. Now, each individual contract obviously will have unique elements, so perhaps Carson's looks much different. But my guess is it doesn't, it would be a disaster if players could just retire then unretire and play elsewhere.
Translation: Carson better be prepared to retire for good if he choses this route as Mike Brown will out stubborn any man on Earth if Carson thinks can just lay low for awhile and pop back up on the West Coast (note: I really hope this isn't Carson's plan).
My hilarious dream scenario: Carson retires for like 4 years, the contract remains frozen, then Carson unretires after getting all fat and letting his skills totally atrophy just to make Mike Brown pay his ass the remaining money.
Of course, there are clearly things built in to prevent this type of nonsense I am sure but that would be pretty funny.
Update: along the lines of a fat Carson unretiring in 4 years to gouge Mike Brown, lazy WDR contributor Comrade Klinglered points me to Cincy Jungle where they note that Carson can somewhat hold Mike hostage with threats to unretire and force either a) a huge cap hit or b) massive cuts basically whenever he wants. Of course, cheap ass Mike Brown may choose to carry a ton of space under the cap to plan for this because he is a dick. Cincy Jungle calls this the "Favre Retirement Strategy". Named after another well known dick, Brett Favre, who did something similar to the Jets. Anyway, point is Carson may have more leverage than I thought about getting out of town.