Hi, Dave
According to the information I have, Knier was one of a few german pilots that were in fact returned from Soviet captivity. I think the idea was to make these guys spies for the Soviets, but I do not believe they got to stay at the northern front very long after their return.
The hard facts:
Bf 109F-4/trop WNr 10145 coded 'Yellow 3' was shot down in aerial combat with multiple soviet fighters during an escort mission for Ju 88 bomber aircraft on July 19th 1942. According to the report from the Gruppenkommandeur Horst Carganico, 5 aircraft from 6./JG 6 started for this mission, the target being Rosta. About 15 kilometers north of Murmansk, the aircraft of Knier was crippled by a P-40 and he bailed out at about 3000 meters altitude.
After being captured by the Soviets he was returned to the German lines and came back to his unit on July 27th 1942.
http://www.ahs.no/ref_db/lw_loss_pub...?lossid=111326
Regards,
Andreas