update item text(RSEG-SGTXT) in invoice

hi,

we are having a requirement to update the item text(RSEG-SGTXT) depending upon other values while creating invoice. is there any badi's or user-exits to update. i tried using BADI INVOICE_UPDATE and user-exits, but none of them is used.

(requirement: when we create a invoice with reference to GR , then the GR text should be copied to each invoice item text)

please suggest me what to do.

Thanks.