Short Answer: The United States had a lack of money, while Germany had too much money (Stagnation vs. Inflation.)
Step-by-step explanation: The United States was supporting Germany so they could make their WW1 reparation fees to the French as stated in the Treaty of Versailles.
The stock market then crashed in America and sprouted panic thru Wall Street causing investors to get completely wiped out and banks to fail cause they weren't insured yet. Everyone went into a panic and consumer spending/investment dropped. The US' Industry declined and they could no longer support the Germans.
Meanwhile, In Germany they soon couldn't pay the French due to no American support so the French occupied their best industrial zone, the Ruhr. The German Ruhr works went on strike and that halted the economy so the current ruling government (The Weimar Republic) decided to print more money to fill the gap. So much money though, in fact, they suffered from hyperinflation. It used to only take 48 German Marks to exchange for 1 U.S dollar, but after hyperinflation in Germany that skyrocketed and the U.S dollar was worth 4,210,500,000,000 German Marks.