get creative with a global (via MV45ATZZ) or static (via custom class) internal table to remember the manually entered routes per item. use USEREXIT_MOVE_FIELD_TO_VBAP to apply code to store/replace the route.don't forget to use USEREXIT_REFRESH_DOCU...
it depends on your detailed functional requirements.Anyways, BSID is the FI table for accounts receivables open items, and it has date fields on it. If you are an ABAP developer then thia should point you in the right direction.
