From a narrative standpoint it's honestly just a bad idea. If Sabo wins then he's overhyped past Luffy's power level and it puts into question the power level of an admiral extremely early in the second half of the series. If Fujitora wins Sabo gets possibly killed or captured(Ace pt. 2) which would be frustrating and bad storytelling.
What is more likely to happen is that Sabo will explain his relationship with Luffy and appeal to the obvious conscience that Fujitora has. Fuji will decide that allowing Luffy to defeat Doflamingo is the lesser of two evils and order his men to stand down. Fuji is waiting for any excuse to get rid of Doflamingo and I believe Sabo will be persuasive enough to allow Fuji's moral compass to trump his duties.
He's already done it once with allowing Luffy to leave the casino after he knew who he was and not telling him he was an admiral. He did it again by trying to give Law an out when he first faced him, he asked if Luffy was his subordinate because if he was he wouldn't be breaking any rules and wouldn't arrest him. Fuji has a track record of doing the right thing and giving people breaks before he goes all in, I expect the same will happen with fighting Sabo or chasing Luffy.
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