Authorization on change of PO ITEM TEXT

Hi.. Experts,

     I have a requirement like, I have to put authorization on some TEXTS at PO ITEM level. But I'm not getting how to put authorization on text, like only      HOD of MM department could change that text.

     I got a BADI -  ME_PROCESS_PO_CUST     Enhancements for Purchase Order Processing: Customer

     and a USER EXIT - MM06E005 - EXIT_SAPMM06E_012

     But I'm not getting what to write there??????

Thanks......,

Sombhu