I haven't read the FIT for Germany, but typical FITs only pay when energy is delivered. If the wind stops blowing, the generator is not able to charge for the capacity of the facility during that time.
Power exports are highly volatile and depend on who else is generating at the time and what the demand is at that time. If the majority of exports occurred in the evening, the exporter is likely doing so at or below cost. However, daytime rates are often several times higher than the price at night which would be a gain for the exporter.
The bottom line is that renewables make power trading more volatile but not necessarily more expensive. More generators mean more competition and the potential for lower prices. The power system is very complicated and very hard to characterize.