What constitutional right gave Lincoln the right to free the slaves?

Answer 1

The Constitution of the United States of America made the President, the Commander in Chief of the Armed Forces. Lincoln was the President. As such he warned the enemy to surrender or that as part of their property that he would take would be their slaves. Armies have always taken and used their enemies' property. What he did as Commander in Chief of the army maintained a great tradition.

In the past, many armies had taken their enemies women with their other property. Since Southerners considered their slaves to be their property, such would be the property which the army would take if the South did not surrender.

So. Lincoln had that right because slaves were the property of people fighting against the United States.