MM-Invoice: Update/Insertion of accout assignments

Is anyone out there who knows how to update/insert accout assignments of mm-invoices? I work with a 46C system and tried desperately to do this via function module MRM_INVOICE_ACCASS_SAVE and direct table update of RBCO but never succeeded. Every time a tried the last way I got an error while saving the invoice.

Pls. help, I am going mad about this issue!

Thanks a lot in advance for any helpful hints!

Alexander