Can you reduce his base salary without pushing the difference into guaranteed money, which has to be paid out at a later date?
Yes.
If the team asks Demarcus Ware to take a pay cut, it would reduce his current base salary ($12.25M) to a lower number, thus lowering his cap number.
They wouldn't be giving Ware a new signing bonus, so it wouldn't change anything in future years of the contract.
If Dallas wants Ware to take a pay cut, they will most likely ask for base salary reductions this year and next ($13.75M).
Ware's contract is written to void following the 2015 season, the final two years were added in 2013 to allow for maximum signing bonus proration.
If Ware agrees, his 2014 base salary will probably drop to something like $5.25M, saving $7M, and his 2015 base salary to $5.75M, saving $8M.
In return for doing this, the base salaries would become guaranteed.
The 2014 base salary would be immediately guaranteed, assuring Ware he would not be released this year. The 2015 base salary would become guaranteed early on in that league year. The Cowboys would be able to release Ware before the guarantee took effect if his 2014 play continued on a downward spiral or if the injuries continued.
Still, the chance to $11M the next two years is probably a lot more than he could get on the open market with another team.
I think it's more than fair.
If Ware refuses, the team has to be prepared to release him.