What was the problem Major James Butler was faced with when the three slaves rowed across the river

He had a problem because according to law he had to return the three slaves back to the southern states. If he decided not to then he would be going against the law and the constitution according to how it was interpreted at the time. If he did return them, they would just be made to work in building weapons for the confederacy as they were before they escaped. He decided to detain them using a loophole in law. Since slaves were considered as property in the south, he treated them as such and since they were a part of the war effort as if they were a gun or a canon, he could detain them since it was legal to confiscate opponent's equipment and use it.