Bwahahahahahahahaha! I guess you have some Clintonian view of the word 'all'. The Confederate commsisioners were not limited to enumerated powers, Davis wrote Lincoln that they were charged 'to agree, treat, consult, and negotiate of and concerning all matters and subjects.' The provisional Congress had commissioned them 'for the settlement of all questions of disagreement between the two governments upon principles of right, justice, equity, and good faith.'
But first of all Lincoln had to accept the southern ultimatum and recognize the legitimacy of the southern secession. The Northern side of the issue was not open for discussion, only the southern demands. Given that the vague offer to discuss issues of disagreement rings very hollow.