This window appear due to party wise special rate define in item to resolve this issue please follow the certain bellow steps:-

 1. Go to Masters > Rate & Discount Master > Party Special > Party Price List > All Items > Party selection window will be appear > Press enter on selected party(in which party purchase selling rate window is appearing) > All item list will be appear > Remove the special rate which is define in window > Then press esc key for save.  

2. Go to Master > MargSetup > Control Room > Search in all > Search -- "Special Rate" > Click on Accept > There is one option shown > "Loss will not effect on Special Rate Items" ---Change that option as "N" > Then press esc key and save.