You approved to purchase for 1000$ - then the PO was reduced to 500$ still the approval is 1000$, then the PO is increased again to 800$ which is still below the already approved 1000$, why would you bother the approver to approve this PO again?

Just imagine, he did not know about the silent reduction, why should he approve something which is already approved for an even higher amount?

you can customize your release indicators to not allow changes in a purchase order after it has been released (changeability = 1)

This way you would force your approver to revoke the release instead of just telling the buyer to reduce the PO.

And again the buyer would not be able to change the PO anymore after it was released.

Yes, I understand your point, and it`s the way we have been working until now. Having said that we would like to avoid that the user has the ability to increase amounts out of his own will without the approver being informed. Lets say the approver has given instructions to deduct the PO from $1000 to $500. A user would then be able to increase back the amount from $500 to $800 for example without the approver being informed. I do not know if there is is a way to avoid such risk using standard SAP configuration.

System stores the information of release value in field EKKO-RLWRT. So when you decrease the amount in PO, system doesn't update the field's value. When you release a PO with amount 1000, system updates the field RLWRT value with 1000. But when you decrease the PO amount as 500, system doesn't change the field RLWRT, it remains 1000. Check the reply from an SAP employee in this thread how reset release strategy works with this field's value.