I think you're blowing this Donald angle out of proportion.
The only way Suh got his money is if he was nearing an extension and then all of a sudden the Rams no longer have money for him.
But by all accounts there is no extension in sight. And even if there was, the Rams could structure Donald's contract where it affects the 2019 salary cap, not 2018s.
Rumor is that the Rams were a finalist in the Tyler Eifert sweepstakes with a 1 year offer of $8M. They lost out. Essentially what would be happening here is they would be allocating that money plus a few mil. to Suh instead.