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Sounds like a SIA, as long as the Marshal is otherwise acting sympathetic as well in the story. Simply stating his sympathies doesn't qualify him. The other criteria seems to be there (particularly the lawbreaking protagonist).
Other tropes that might apply depending on the story details are To Be Lawful or Good (in this case, hes choosing lawful), Lawful Stupid, and Bothering by the Book (the last two if he's upholding the law even if he could justify not in some way, such as Loophole Abuse).
Edited by BeerBaronBothering by the Book is specifically a form of protest, by following all the rules in a way that's meant to obstruct the system. So I don't think it would apply here, unless the privately pro-abolition Marshal is recapturing escaped slaves solely as part of a 5-D chess scheme to abolish slavery.
A less sympathetic portrayal could also paint him as My Country, Right or Wrong and/or Just Following Orders, two forms of choosing to carry out the orders of an authority over one's own conscience - the former if he's the one in charge locally, the latter if he's under the command of someone else

In a book where the protagonist had just helped with the Underground Railroad, a local Marshal says:
Could the Marshal in question be categorized as a Sympathetic Inspector Antagonist ? If not, what trope would fit him?
Edited by Someone1981