This puzzle is intended to make a point, but it doesn't because the solution is so simple:

1. I return the 100 pennies to you.
2. I buy (or trade for) one penny from you.
3. I give you the one penny interest.

Here is another solution:

1. I loan the 100 pennies back to you + interest.
2. The loans are the same, so we agree to cancel them.


The point the OP is trying to make is that if money is debt, then interest cannot be paid if there is only enough money to repay the loans. According to the OP, the U.S. must go deeper into debt in order for there to be enough money to pay the interest, but that means yet more interest that cannot be paid.

Since all money is backed by debt, there seems to be a problem because if all debt is repaid, then there is no more money (to pay the interest, for example). Of course, there is no real problem (other than having no form of money), as I have pointed out the solutions above.