U.S. debt is issued by the Treasury in bonds, bills, and notes depending on maturity. Treasuries are sold and then enter into the secondary market where, depending on the type of Treasury and the return and the duration remaining and economic conditions, they can trade for more or less than face value. If you are trying to buy back Treasuries with dollars worth less than when they were issued you can expect to pay a premium. So to retire $X trillion in debt you will likely pay $Y trillion where Y reflects the premium paid.
What would cause their market value to rise relative to their nominal value if the dollar is devalued by inflation?
Why would we not be buying back the nominal amount with dollars that are now worth less?