I need to add a very significant caveat.
Understand that "signing bonus" does not appear to be a CBA term. The CBA talks about Salary and Guarantees. With respect to earned Salary the CBA is very clear that no team and player may agree to any contract that voids the earned Salary except in very specific events which do not apply here -- incarceration, retirement, missing practices for hold out.
So, if the $9 million is regarded as earned Salary, I think the Patriots may have a significant problem. Put another way if the contract really makes it earned on signing and it is salary and not a guarantee, and the date of payment is just an accounting thing, then they may have a tough road and the stuff I posted above may not apply in this case.
I tried to give it my best reading. But an agent or player rep may know what these words mean off the bat and I have never seen a player contract.