I have never run across any texts that claimed that. I know there were black units before the EP (Kansas volunteers and some "contraband" units on the Sea Islands in Georgia enlisted by General Hunter I believe and against War Dept. policy) but I have never seen anything that offered freedom for enlistment. Do you have a source?
I read about it in a history of the war in Maryland from the Confederate point of view. It could be the claim was false that slaves were offered their freedom to enlist.