Hi Ivan,
I am totally agrea with Andrea, Customer Posting will be done through operative valuation area by the configuration suggested above and by assigning posting specification of UPDATE Type and account symbol to posting spec and account symbol will carry recon account.
Secondly,
I business partner Under Customer general tab click on Create FI Customer and create customer link with business partner. After that while creating contract check uner partner tab, system is showing customer number linked with business partner. If it is coming. Then take action on loan and try to post accounting entry.
I hope this will resolve your issue.
Regards
Narinder Pal Verma/