Steal? Who says? Lincoln could have sent negotiators to decide how much of the standing navy, army, western lands, etc. rightfully was due to be turned over to the South (in addition to the obvious areas within their borders), but he wanted war. That’s aggression.
The slavers had no desire to negotiate - they were all about making belligerent demands. There was no reason why Lincoln should be compelled to negotiate - but he did try anyway. His attempts were rebuffed by the belligerents.
He didn't have to, it is historical fact that the South sent a peace delegation to DC that Lincoln totally ignored.