I'm gonna state the obvious, as looked at through the eyes of the times. (<--key)
So.... the Confederate Army was supposed to get one of their Soldiers back, in exchange for giving up a runaway slave..? ie: their own property that was either, stolen, or had escaped, & joined the fight against them..?
Imagine, I steal your property. I then use your property against you. How willing would you be to, exchange your own property back to me, to use against you, in exchange for your own property ?
Think about that from a practical standpoint. It was purely a win win for the Union side only. The Confederates were expected (in some cases), to give their own property back to the Yankees, in exchange for one of their own soldiers.