That's actually not the case at all. The advance was on guaranteed salary.
Advances, as the post I gave your for Manning, are treated as bonuses and prorated the same way. They are not treated as an unpaid guaranteed.
I do see that you are right though on the guaranteed salary part. Good catch, I missed that. When Tebow signed the contract the salary was guaranteed. However, the next year he took the 6.2775 million in guaranteed salary and turned it into an advance, therefore changing it from a salary to a bonus from a cap hit standpoint.
From an accounting standpoint even if Denver were willing to pay part of the guaranteed salary it would still be salary and the responsibility of the team who's roster he is on each week. I cannot find anything stating that a team can pay the salary of a player on another teams roster (cap hit wise).
The only way I can see to do this would have to include the player and a renegotiation of the contract to turn that guaranteed money into a bonus or advance shifting its cap accounting. The bonus would then be Denver's responsibility and they would have to negotiate with us to pay some of it. There are a lot of moving parts to that.
Again, I could be missing something, but while I see no verbage that teams can't negotiate this, I also see no verbiage stating that a team can agree to assume the cap hit for the weekly salary of a player on another team (which is what would be happening in this case).
His Base Salary is 7 million. The original guaranteed money was 20 million.
He was paid 5 million in signing bonus, making the first 15 million of his contract guaranteed.
That takes you into 2016.
He got a $5 million signing bonus, a $5 million roster bonus, and a base salary of $3 million last year. His cap hit for this year is actually $1,666,666 more than his salary would be. So they would save money if they release him (though less than $200,000), nothing guaranteed would carry to 2106 except the prorated portion of his signing bonus (which was already paid) and of course the $10 million in non-guaranteed money (base salary and roster bonus) if they keep him.
http://www.spotrac.com/nfl/denver-broncos/demarcus-ware/