Frank, I found this correspondence interesting for a couple of reasons. First, it illustrates the concerns of the slave-holding planters in southern Missouri . Second, and most interesting, is the opinion of a U.S. departmental commander, at this very early stage of the war, of what the war was NOT about. General Harney had received no special instructions or briefings from Washington about the issue of slavery. At his command level, he would certainly have been privy to discussions concerning the war aims of the government. This, and other correspondence from around this period of time, demonstrates that the United States was pursuing the war to restore the Union, not to free the slaves. That changed, of course, later in the war, when it became expedient to redefine the aim of the war in order to deter foreign governments from recognizing and supporting the Confederate States.

And you're right that this early correspondence reflects Lincoln's ambivalent position on the subject of abolition.