Change line item value while saving a PO

Hi All,

I have the below issue -

While creating a Purchase Order in ME21N, except the PO item with the highest net price, all other line items should be locked ( EKPO-LOEKZ = 'S').

I have tried using EXIT_SAPMM06E_013. I was able to modify XEKPO value within this exit. But later when the PO is saved, the lock is not reflected.

I tried using ME_PROCESS_PO_CUST, in method check/post, but the item values are not updated.

I also tried ME_PURCHDOC_POSTED, but i am not able to modify IM_EKPO value with this.

Similar issue within EXIT_SAPMM06E_017.

Please advice which exit / BADI can I use for this case?

Thanks

Meenakshi