Why did the germans allow lenin and his associates to return to russia?

To overthrow the government and seize power.
The Russian communists and poverty-stricken were preparing a rebellion in February 1917 when Lenin was in Switzerland and the German Emperor decided to take advantage of the civil unrest. The Russian government was occupied trying to control its people which meant they had less resources and men to fight the Germans.

He would most likely post his laws in the city market