That was exactly the argument made by pro-slavery advocates. The Constitution guarantees property will not be confiscated without due process and just compensation (think imminent domain). Slave holders argued that seizing slaves via freeing them without compensation was illegal. But remember than Lincoln was an attorney before he was a president. The Proclamation was designed to not be a violation of the Constitution. It only includes the "areas in rebellion" and specifically left out the border states and dozens of counties in VA and LA. The underlying argument in a legal sense was, if you (Confederates) are using slave labor as part of the war effort (and they were: trenches, cotton-picking, factory labor), then it's within our right as the opponent to capture said goods as "contraband of war" and use it for our side. No different legally than guns, horses, wagons, or cannons. Lincoln allows for their freedom, but almost of equal importance, allows them to enlist, and some 180,000 would eventually fight for the Union. Some of the newly formed regiments were even referred to as "contraband" regiments.