wrong a negative feedback from a buyer ,,,

Hello ,

i were listing some items in the market and someone buying a two items from me ,

My items a digital code and i were making my items is automatic delivery ,

Now , the buyer give me a negative feedback two times without any reason !!

he says in the feedback good seller , fast trades , but he mark as negative feedback !!!

Please Help me in this issue to change a feedback Because i’m very sad for that ,

My Code is : NDNWRH