What caused Lincoln to fear that freeing slaves would mean the dissolution of the Union?

Answer 1

No.

The only danger would have been if he had declared that all slaves, North and South, should be free. The slave-owners of Missouri, Kentucky and Maryland might then have taken their states into the Confederacy.

Answer 2

Quite understandably, US President Lincoln and others believed that if four million slaves would immediately be freed at once, there would be economic chaos. This is said remembering that prior to the US Civil War, Northern textile mills used Southern cotton and the North also traded with the South on a mutual and orderly basis. When Lincoln was alive, the so-called border states in the Union kept their slaves. Throughout the war, he tried unsuccessfully to present to these states comprehensive and compensated plans to gradually abolish slavery. These border states refused any such system.Lincoln believed that if the Union-Northern states and Congress created an amendment to the US Constitution abolishing slavery, the Union's border slave states would join the Confederacy. Using that line of reasoning which was sound, then yes freeing border state slaves could and or would lead to a stronger Confederacy and spoil any plans to keep the US united as one nation.