Not sure what you are referencing by talking about cutting a player and dating the move backwards? Plus not sure where dead money is a key component of the question.
Let's make this simple. Let's say Philly is $4 million over their cap right now and have no other moves to make. The day before March 17th, they trade Ertz and the end result is that trade would save them $5 million on the 2021 cap. So by that they'd have $1 million in cap space putting them in compliance with the league rule that they must be under the cap by 4PM on the 17th. Per CBS Sports, all teams must be under their cap by 4PM on the 17th:
However, that trade is unofficial and doesn't become official until the league year starts. Per the same CBS article:
So technically Philly was not under the cap prior to 4PM on the 17th. They didn't get the cap space recognized until after the league year started. Does the league essentially retroactive the savings to prior to 4PM, thus giving Philly the cushion they needed to be in compliance with the cap rule?
I once thought that was the case but now I have read some things that seem to indicate maybe not.