If the Rams slapped the franchise tag on Fowler they would immediately take a cap hit of over $16M. And if they signed Littleton they would take another huge cap hit. (The exact amount would depend on the structure of the contract, but $13M to $16M wouldn't surprise me.)
As things stand the Rams can't afford to sign Littleton and tag Fowler.
In addition, if the Rams do figure out a way to tag Fowler they can't trade him unless he signs the offer. My guess: he won't sign the tag offer until a.) the Rams work out a trade with a team that Fowler approves and b.) his new team offers him a long term contract to his liking.
AlbaNY_Ram