I think what you and Teegre are envisioning is not kosher in the NFL, but I don't know. This is the clearest explanation I have seen anywhere to date and I can not vouch for it's absolute correctness:
"Now, should the Steelers choose scenario No. 5 above, they would have the previously noted dead money charge in 2019 of $21.12. If you do the math, that’s just $1.045 million less than his cap charge would be should they choose scenario No. 1, which is keeping him. When you factor in roster displacement cost of trading Brown, the Steelers really wouldn’t save very much 2019 salary cap space by going with scenario No. 5. They would, however, save $15.125 million in cash in 2019 along with the remaining $23.8 million Brown is scheduled to earn in 2020 and 2021 and in the process rid themselves of one very, very, very talented headcase."
https://steelersdepot.com/2019/01/st...antonio-brown/
So trading him in the narrow March window costs roughly $21 million in 2019 cap space but something like only $6 million in actual cash money. I think this is where the confusion comes in for everyone thinking and writing about this. There is the actual amount in currency that the 2019 Pittsburgh Steelers would owe Antonio Brown and then there is the amount of cap dollars the team would owe the NFL accounting rules.