See this is where the whole proof thing would normally be required. This situation was reached by people taking risky decisions that seemed like a brilliant idea at the time but were completely unsustainable in the long term, I'm of course talking about subprime mortages.
For those who aren't aware, the idea is to lend money to those who normally would never be allowed credit, i.e. those that are sub prime or under-banked. However lenders needed something to back these mortgages up, they weren't going to just give money to people who most likely wouldn't be able to pay them back, so they did a dirty little trick. They realised that the economy was incredibly strong and so they sold the debt to another bank, they get their money back, the next bank can then sell the debt on at a higher price because of the interest that has acrued. The idea should've been that the original bank could buy back the debt once those who borrowed the money in the first place had paid it back with interest. This amount would be much greater than the value the debt had risen to by that time. The problem is, this only works when the economy is strong, when people can afford to pay their bills and banks are confident to lend to each other. If banks aren't lending then the debt doesn't get back to the right place. These are the debts that the government are trying to pay for with the bailout bill.