I recently found out that my great grandfather used POW as farmhands on his estate during the war. He had a lot of land to work and I can imagine that at some point in the war most of his German staff was drafted.
So how did it work? Did business owners write letters to some ministry asking for forced laborers or were they approached by the authorities? Could they just have closed down their business and did something else because they didn't want to participate in this slave economy?
Could a farmer, for example, just sit on his 100 hectars, retiring and not working the land, just because he didn't want to use POWs? Especially when food was of course important during the war.
Firstly, the Geneva Convention of 1929 did allow a detaining power to use POWs for labour[1]:
"Belligerents may employ as workmen prisoners of war who are physically fit, other than officers and persons of equivalent statue, according to their rank and their ability. (…) It is forbidden to employ prisoners of war on unhealthy or dangerous work."
In Germany, the way to request POWs for labour would be through the local labour office (ger: Arbeitsamt)[2]. The labour office would then send the request up the chain to the regional labour office or even the Department of Employment, where the distribution of employable POWs was planned. Then, orders were sent to the POW camps to send detachements along with guards.
Usually, the municipality would collect the requests and ask for POW detachments of appropriate size. The smallest size of a detachment for farm labour was 10. For other employment, the minimum size was 20, however local employers would pool their requests. Factories would get their own detachements.
It seems that there were far fewer employable POWs available than there was work for them.
Sources:
[1] International Committee of the Red Cross (ICRC): Convention relative to the Treatment of Prisoners of War. Geneva, 27 July 1929, Art 27-34.
[2] Runderlass des RMdI vom 12.8.1940