User exit at PO Save (ME21n)

Hello,

I want to change the valuation type of the Purchase Order item when I save a PO created via ME21n based on checks.

For this which user exit should I use? I tried EXIT_SAPMM06E_013 of enhancement MM06E005. But that didnt help.

I want a user exit which triggers when we save PO and should enable to change fields in item level. The saved PO should show the changed values.

Any thoughts??

Thanks

CMV