When Supplier sold the item less than the actual value of item then user will make use of credit note in price diff purchase.

Go to Transactions > Price Diff. Purchase > Credit Note > Purchase price diff credit note entry window will be appear > Feed date > Select party > Bill details window will be appear > If user remember the bill number of credit note then enter that number here or press esc key > Press enter > Select item > Next a window will appear at the bottom of the window where we have the old rate or new rate field > Enter Old and New rate(Actual amount) > Then press enter key and it will show price diff on the bill >  Press Ctrl+W for Save.