User exit for PO Item data update

Hello All,

At time of creation of PO using ME21N / ME58, we want to update PO item data . We want to update Over and Under delivery tolerances values from corressponding sales order rather than purchase info record.

Please let me know if anyone has worked on similar requirement or any pointers for relevant user exits in SAP Release 4.6C .

Regards,

Manu.