I found an interesting discussion of southern debt in a 1901 book, The Confederate States of America, 1861-1865: A Financial and Industrial History of the South During the Civil War, by economist John Christopher Schwab. He doesn't specifically mention the use of chattel slaves as collateral, but he does have some interesting things to say about the level of southern debt and its possible role as a motive for southern secession. Much of his chapter on "The Southern Debtors" is about stay and sequestration legislation in the Confederate states, but here are some quotations that seem especially relevant to the discussion here:
"The scanty records in the South do not furnish a sufficient basis for an estimate of the extent of the debtor interest during the Civil War. That it was as persistent and as effective in procuring desired legislation as under previous similar conditions is a fair inference from the adoption of the above number of stay laws.
"Another closely related question, the solution of which we can also reach by inference alone, is this: Did Southern indebtedness to the North add strength to the secession movement in 1860 and 1861? That it did, the following paragraphs would indicate; how much it added to the movement it would be rash to guess. The same questions are involved in the causes of the American Revolution, among which must certainly be enumerated the desire of the Americans to avoid the payment of their debts due to Englishmen; how weighty this motive was no one can say.
"A full month before seceding from the Union, Georgia passed a stay law, in which it was provided that no levy of attachment should be allowed unless the claimant declared under oath that the defendant was about to remove from the South or any county; Mississippi and Alabama passed stay laws within a month after seceding, and the Virginia Convention was putting difficulties in the way of Northern merchants collecting their debts in Richmond at the time the State seceded. These may fairly be styled suspicious circumstances; but will be passed over as the necessary concomitants of the commercial collapse of those months, itself caused by the fear of secession.
"We are very much in the dark as to the extent of Southern indebtedness to the North at the beginning of the war. Some put the amount as high as 400 millions of dollars, and the debts in New Orleans alone at 30 millions. Others accepted the estimate of the New York Tribune, namely, 200 millions, made in September, 1861, when the various Confederate sequestration acts were already in operation, and had unduly magnified the importance of the Northern interests involved. The most careful estimate at our disposal was made by the United States Economist. The writer anticipated a general repudiation by the South of debts due the North. A large amount of such debts had been contracted in the spring of 1860, at a time of buoyant feeling; the general depression of the fall of that year had compelled the Northern creditors to frequently renew such loans, and they were still unpaid when hostilities broke out. At that time the Economist estimated the outstanding indebtedness of the South to the North at 40 millions of dollars, three-quarters of the usual amount in the spring of the year. New Orleans, Savannah, Mobile, and Charleston had bought only two-thirds the usual amount from Northerners in the fall of 1860; and during the following months, as the clouds gathered, credit was but sparingly given to Southern buyers. Unquestionably the Northern creditor was not caught napping, and had prepared himself for the storm more thoroughly than some of the above estimates, which seem exaggerated, would indicate.
"... The cotton of the South was moved by drafts upon New York and London. The Northern and English banks advanced the desired capital in the shape of currency to the Southern cotton factors and planters. The latter keenly felt this 'abject banking dependence,' and hoped to escape it when they seceded from the Union. It is noticeable that the distinctively cotton States were the first to secede, that all of them did so before President Lincoln's inauguration, and all of them, with the exception of South Carolina, —which had led the movement on December 20, 1860,—within twenty-three days thereafter. We shall see how the complementary notion of the industrial dependence of the North and Europe upon the South was fostered by the conditions the war brought about.
"To return to the indebtedness of the South to the North in 1860: It must have been these bankers' advances upon the cotton as it moved to the Northern and English markets which were the basis of the extravagant estimates cited above. Moreover, it is to be remembered that the Northern banks could not have suffered seriously by the repudiation of their claims upon Southern cotton men on the score of their advances, as they held sufficient collateral security in the cotton, which was practically consigned to them.
"Even with the above qualifications, the debtor interest in the South must have been of sufficient weight to be taken into account among the factors which led up to the formation of the Confederacy."
(John Christopher Schwab, The Confederate States of America, 1861-1865: A Financial and Industrial History of the South During the Civil War, New York: Scribner's, 1901. Pages 106-123.)