I'm looking for an instance where we can decisively point to as the US Federal government using black soldiers in formal combat against white adversaries. A specific date or battle would be appreciated if possible.
I assume it happened sometime during the Civil War right?
Hi OP! I hope I can help, if not I'm sure the comment will be removed which is understandable.
Joseph Ellis, author of "His Excellency: George Washington", notes how Washington actually lifted what was initially a ban on black enlistment within the Revolutionary Army (during the American Revolution) in December 1775 for blacks that had already been in the army.
This is notable because in July of the same year, Washington had sent a letter to recruiters telling them not to recruit "any deserter from the Ministerial army, nor any stroller, negro or vagabond unable to bear arms, nor old men unfit to endure the fatigues of the campaign, are to be enlisted." Beforehand, and this is where my knowledge is limited without my library, slaves were allowed to enroll in some militias.
By December, Washington was under great pressure to make big moves against the British and realized he had a severe shortage of manpower. Ellis even notes that the British were allowing black men to fight on their side and were offering freedom to those who fought, so Washington knew he had to do the same. He changed his mind in December and allowed the re-enlistment of blacks who had already served in the army.
Enlistment also depended upon the state. Some states were okay with allowing it, others were against it entirely. Some were okay with allowing freedmen and slaves, others only wanted freedmen.
I apologize but he does not go into much further detail, so I cannot confidently tell you how many or in what role they served. Does this partially answer your question? Your use of the term "US Federal government" can lead to some interesting interpretation so I want to make sure I'm in the right area for you. I do not believe this is the earliest that blacks were allowed to fight against whites under the US government and it's banner, but I believe it is close.
If the mods are okay with this post and you have further questions, please feel free to ask. My degree focused on American Colonial history and I have quite a few books at home that might be able to answer your questions- I'd be happy to look when I have the time (and I'll see if I can find references that note earlier enlistment of blacks).