They could of fought at UFC 254 I think, right after Dustin fought Hooker and Tony loss to Gaethje.
I think there was a hold up on Dustin's side for compensation, Tony at the time backed him up on it.
From what I remember and I might be wrong so bare with me, but basically they were trying to get the Ferguson and Poirier to also be set up as replacement fighters for the Khabib/Justin bout.
These were the lined up scenarios:
Justin pulls out, they do Khabib/Ferguson(leaving Dustin without a fight)
Khabib pulls out, they do Justin/Poirier 2(Leaving Tony without a fight)
And I'm guessing the the possible issue Dustin had is he was getting extra compensation for also being a possible replacement fighter or in another case, not fighting at all in the scenario where Justin pills out.
Also I think this is where Dustin's dislike for Chandler began because he basically stepped up as the replacement guy for Khabib/Justin which most likely messed up his negotiations.
Anyways this fight would of been fire in like 2017-2019 space, but Tony's washed as hell and needs to retire ASAP.
Dustin would murk him at this stage with ease.