Solution : https://service.sap.com/sap/support/notes/1050808 (SAP Service marketplace login required)
Key words : 
call loan contract rfc call function 'bca_in_acc_sl_lc_get_balance'   destination l_str_rfcdest   exporting     standard_id       = l_bca_iban importing    loan_cap          = l_str_bca_cap_list    log               =, rfc destination     call function 'cms_api_rfc_dest_get'       exporting         i_rfc_appl          = con_rfc_appl       importing         e_str_rfc_dest_data = l_str_rfcdest, call function 'conversion_exit_alpha_input'       exporting         input  = l_str_rbl_sky-rbl_id       importing         output = l_bca_iban, ->map_acc_fields         exporting           im_str_acc_balance = l_str_bca_cap_list         importing           ex_str_rbl_hdr     = l_str_rbl_hdr, read loan contract key figure class, read current account key figure class, call rfc bca_in_acc_sl_lc_get_balance, shift l_rbl_id left deleting leading '0', call rfc bca_in_acc_im_get_current_bal, internal structure       call method
Related Notes : 
       
| 1050530 | Missing RFC for determining the loan capital amounts | 
| 1049351 | SAP Banking Services 5.0 SP02 | 
| 1024091 | Release limitations concerning SAP Banking Services 5.0 |