This is true. The “pie” is dynamic in that it changes with the league’s revenue, but at any moment in time it’s the same for every team. Once a team commits to an individual player’s compensation they’re locked into the terms of the agreement.
Teams may build into agreements flexibility allowed in the CBA. Example is what the Bengals are wanting to do with Shemar Stewart. Most teams have used the same language in their first round draft picks contracts for years. He’s balking. Conversely, right now 2nd round picks are not signing trying to get guarantees not generally agreed to in the past for 2nd rounders. (30 of 32 have not signed)
The great trap is that teams hope to acquire a great talent to make the team competitive for a Super Bowl. Signing that great talent means other talent will have to go elsewhere to “get theirs”. The Eagles are a perfect example. Their vaunted defense on a Super Bowl winning team lost 4,000 defensive snaps from last season through free agency, trade, retirement. There is no way (no way!) they repeat this season while they plan on how they’re going to afford to pay Jalen Carter. That’s the way it works. Philly will sign him immediately when they can after this season and not wait until he’s going into his fifth season. That’s the roster building some owners don’t grasp that you’re talking about, I think.