I just read through relevant parts of the CBA, primarily Article 10. It seems a team could renegotiate this years contract to pay him more.
What I do find more interesting is it is quite reasonable for Bell NOT to play at all this year. If Bell sits out the entirety of 2018 and the Steelers franchise tag him again next year, it would count as tagging him for a third time. As such, per Article 10, Section 2b, the tag would be the average of the top five QB salaries. I don't believe the Steelers would tag him for that amount, so Bell would be a free agent.
Perhaps I have it wrong. The text is below.
Section 15. Signing Period for Franchise Players:
(c) If any Franchise Player does not play in the NFL in a League Year, his Prior Team shall have the right to designate such player as a Franchise Player or a Transition Player the following League Year, if such designation is otherwise available to the Team, except that the applicable Tender must be made and any 120% Tender shall be measured from the Player’s Prior Year Salary. If such a player is redesignated as a Fran-
chise Player for the League Year following the League Year in which he does not play, the player may be designated only under Section 2(a)(i) above, except that Draft Choice
Compensation of only one first round draft selection and one third round draft selection shall be made with respect to such player in the event he signs with the New Club. If such a player is designated as a Franchise Player for a third time, the terms of Section 2(b) above shall apply. If a Franchise Player who has sufficient Accrued Seasons to become an Unrestricted Free Agent is not designated as a Franchise Player or Transition Player for any League Year immediately following a League Year in which he does not play, then on the first day of that League Year, the player becomes an Unrestricted Free Agent and is completely free to negotiate and sign a Player Contract with any Club, and any Club is completely free to negotiate and sign a Player Contract with such player, without penalty or restriction, including, but not limited to, Draft Choice Compensation between Clubs or First Refusal Rights of any kind, or any signing period.