Surely the key phrase here is in the original post "Work booked my return tickets through American Express travel".
"Work" presumably have policies and arrangements with AmeEx travel, and a cheap flight was booked based on the poster's instructions.

If the poster needs to change for business reasons, then this would seem to be "work's" problem to sort out, and absorb the price difference.
Or alternatively it is the poster's problem to suck up the price of the return ticket (or changing the existing ticket) if it is the desire of the poster to return early.