Manpower could also be juggled back and forth. This was part of the pre-Barbarossa plans: demobilize workers, call them up for a short Eastern campaign and then dismiss them again.

But the only definitive solution was to increase the number of workers. They could be brought from the several countries under occupation to make up for conscription losses and go beyond that. Besides this simple logic of addition, importing workers also made sense as Germany had the highest productivity in Axis Europe. A French worker in Germany was more productive than a French worker in France. A concentration of the continent’s manpower within the German economy was what any “rational economic dictator” would do.
The usage of foreign labor began with Poles in agriculture. There was already a tradition of temporary work across the border and in 1938, with the harvest looking difficult, Backe had proposed to negotiate with the Polish government a transfer of laborers to German fields. In 1940, with the food supply in danger and the pre-France strategy of total short-term mobilization dictating a greater exploitation of occupied territory, Goering, Backe and Hans Frank (occupation administrator) agreed to use the General Government as a reservoir of labor, employing a large proportion of its population and tapping into areas without traditions of temporary cross-border work.
This clashed with Himmler’s intentions: Germany had just annexed several Polish territories and he was working to have the Poles leave, not enter the country. The arrival of large numbers of Poles and their participation in society were not desired. A compromise was reached in subjecting the arrivals to apartheid, severely restricting their social contact with Germans, aswell as a cap to their wage level and harsh punishments for shirking at work -but never deportation back to the General Government.
POWs were put to work and volunteers arrived, but that was not enough. Manpower targets had to be met by forceful transfer. As this provoked a reaction by the subject population and occupation authorities had little military strength they preemptively targeted the nucleus of a nascent Polish resistance movement, with purges beginning in the summer.

The usage of foreign labor widened, assimilating Western POWs and delving into other sectors such as coal mining. But it truly boomed in 1942, when in response to the manpower crisis, Hitler appointed Fritz Sauckel to the new position of general plenipotentiary of labor mobilization (GBA). Sauckel was a crude, plebeian representative of the NSDAP’s populist wing. He has been compared unfavorably to the “handsome, urbane technocrat” Speer, but that underestimates him and downplays the rationality of his efforts. He succeeded in extracting millions of workers from occupied territories, either voluntarily or through press gangs. To give an idea of how many foreign workers were in Germany:
Between January 1942 and the end of June 1943 the GBA delivered 2.8 million new foreign workers to Germany: the workforce of a great factory - 34,000-strong – every week, for seventy-eight weeks. By the summer of 1943 the total foreign workforce had increased to 6.5 million, of whom 4.95 million were civilians rather than prisoners of war. After the summer of 1943 the pace slowed somewhat, but Sauckel continued to bring in workers. By February 1944 the total of foreign civilians and prisoners of war had risen to 7.356 million. By the autumn of 1944 it had reached a maximum of 7.907 million. At this point, foreign workers accounted for more than 20 per cent of the German workforce. Of the armaments workers of the Third Reich, more than a third were foreign. In the plants of the Reichswerke Hermann Goering and the Luftwaffe, the foreign share routinely exceeded 40 per cent. On individual production lines the percentage could be even higher. As State Secretary Milch boasted in June 1943, the Stuka Ju 87 was being '80 per cent manufactured by Russians'. (p.517-8)