According to legend a certain Prince Leshek(Leszek) IV gave German Jews written permission to enter Poland. What is known is that German Jews did arrive in the late 9th century prior to the Piast Dynasty (960) and preceding its becoming a Christian country in 966.
906 MAYENCE (Germany)
First record of a Jewish settlement in the city.
906 - 1006 SHERIRA GAON (Pumbedita, Babylon)
Served as Gaon from 968-1004 when he appointed his son Hai Gaon to serve in his stead. Sherira wrote a Biblical commentary as well as one on several tractates of the Talmud. He is famed for his "letters" Igeret Rav Sherira, a history from talmudic times until his own time in which he explains the development of the Talmud.