The issue of Confederate debts incurred during the Civil War was settled by the passage of the 14th Amendment, ratified on July 9, 1868. Probably its least-remembered provision reads:

The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.

In plain English: Neither the Federal government, nor any of the state governments, is allowed to pay any debt that was undertaken to aid a rebellion against the United States. This primarily affected the bonds issued by the Confederate government during the war. Those debts were declared "illegal and void" - the bond-holders could not come to the Federal government or any state government to obtain payment on the bonds.