Why less American aid would not free Germany from the Eastern Front

Without American aid to the Soviet Union, could Germany shift more forces against Britain in 1942 or 1943?

Probably not soon; Soviet offensives would slow, but German logistics and manpower would still bind major forces to the Eastern Front.

The counterfactual cannot be decided by Soviet combat strength alone. German operations were already constrained by the distance from their supply base and by the poor infrastructure encountered farther east. American trucks and aviation fuel were important to Soviet mobility, so their absence would prolong the campaign and delay major offensives. It would not remove Germany's own transport and manpower shortages or suddenly release large air forces for use against Britain. As the front approached Soviet industry beyond the Urals and receded from German supply sources, the logistical balance would continue to work against a quick German victory.