The 16-year-old defender has been snapped up before Dortmund could secure him on a professional contract.
He has penned a deal that ties him to Anfield until June 2012 and follows in the footsteps of Christopher Buchtmann, who also joined from the Bundesliga club.
However, Dortmund youth director Peter Wazinski believes Sama has a long way to go before he is the finished article and has told Liverpool he is not such an exciting prospect as Buchtmann.
"This time they did not look closely enough," Wazinski told Reviersport.
"Stephen was already a substitute last season, unlike Christopher Buchtmann was when he joined them.
"In terms of Stephen Sama, the transfer does not hurt us in any way, except that there is always a better moment to join another club than in the middle of pre-season."