Can you see the same amount in item level also ?
If you will change amount in item level also, then system will take the rest 200 amount from your Freight expense G/L (which is showing in PO - Account assignment tab)
If you will only enter the new amount 2500 in header and your item ha the amount 2300, then system will take the G/L account from OBYC - Transaction key DIF.
Although you can assign the price difference G/L account in OBYC for transaction key DIF. But then every this type of small price difference will go to this G/L account.
