The way you pay off that debt is not to the original liberator now turned oppressor, but by extending similar help to countries that are now in a similar bind as we were then. Like Ukraine. I really think we are morally obligated to liberate and help Ukraine.
Our debt to the US has long been paid off. It was paid off when we submitted to their economic world order, when we bought their goods and their entertainment, when we bought their software and let our own software industry dwindle, and finally when we went to war on their side on their questionable military adventures.
We owe the US nothing. I will still help them when they actually want it, but not like this.