Generally speaking they paid money for goods and services, though to be perfectly clear they paid with French money collected via taxes in France. As a general policy of Nazi Germany occupation costs were transferred to the occupied country.

This and other information about the Nazis' methods of economic exploitation can be found in Götz Aly's "Volksstaat", though I'm not sure whether there is an English translation.

The Germans looted the French economy.

France, Belgium and the Netherlands were forced to pay for their own occupation in manufactured goods and other assets, a total of $33 735 000 (in 1938 US dollars) were siezed this way.

The Germans also set the exchange rate of the reichsmark to the French franc about three times as high as it had been 1930. German soldiers, who were paid in reichsmarks could buy astonishing amounts of good and luxuries on a regular soldier's pay this way.

So the Germans paid in money - that were either extracted from the French themselves or set at an impossibly high value through the Exchange rate.

Either way, the French got shafted.