The Counterfeiters

The film begins shortly after the end of the Second World War, with a German man arriving in Monte Carlo. After checking into an expensive hotel and paying with cash, he takes in the high life of Monte Carlo, successfully gambling in a casino and attracting the attention of a beautiful French woman. Later, she discovers tattooed numerals on his arm, revealing him as a survivor of the Nazi concentration camps.
The film then shifts to Berlin in 1936, where the man, Salomon Sorowitsch, is revealed as a successful forger of currency and passports. Caught by the police, he is imprisoned, first in a labour camp, then in Mauthausen concentration camp near Linz. In an effort to secure himself protection and meagre comforts at the camp, he turns his forging skills to portraiture, attracting the attention of the guards, who commission him to paint them and their families in exchange for extra food rations.
Sorowitsch's talents bring him to wider attention, and he is transferred out of the concentration camp. Brought in front of the police officer who arrested him in Berlin, he finds himself put together with other prisoners with artistic or printing talents, and begins working in a special section of the Sachsenhausen concentration camp devoted to forgery. The counterfeiters are kept in relatively humane conditions, with comfortable bunks, a washroom and adequate food, although they are subjected to brutality and insults at the hands of the prison guards. His fellow prisoners have a range of backgrounds from Jewish bank managers to political agitators, and while some are content to work for the Nazis to avoid the extermination camps, others see their efforts as supporting the German war effort.
At first, self-preservation appears to guide Sorowitsch, but his motives for forging for the Nazis are complicated by his growing concern for his fellow prisoners, his awareness of their role in the wider war against the Nazis, and his professional pride in counterfeiting the US dollar, a currency he was previously unable to forge.
Sorowitsch juggles the demands for progress of the Nazi authorities, his co-counterfeiters' determination to sabotage the operation, and his loyalties to his fellow prisoners. The prisoners successfully counterfeit the British pound but intentionally delay the forgery of the US dollar. Gradually, slivers of evidence that the war has turned decidedly against the Nazis arrive. One day the camp guards suddenly announce that the printing machines are to be dismantled and shipped away, which leads the counterfeiters to fear that they will finally be killed. Before anything happens to them, the German guards flee the camp in advance of the Red Army. Starving prisoners from other parts of the camp, armed with confiscated weapons, take over and break into the compound where the counterfeiters had been held in relative luxury. Until the insurrectionists see the well-fed printers' prison tattoos, they believe them to be SS officers and threaten to shoot them. The counterfeiters then must account for their forging actions to the half-dead prisoners.
The film then returns to Monte Carlo where Sorowitsch, apparently disgusted by the life he is now leading on the currency that he forged for the Nazis, intentionally gambles it all away. Sitting alone afterward on the beach, he is joined by the French woman, concerned after his seemingly disastrous losses at the table. Dancing slowly together on the beach, she consoles him regarding all the money he has lost, to which he replies, laughing, that we can always make more.

Question: What currency was Sorowitsch previously unable to forge?
Answer: US dollar