Cant edit pricing on a listing

I have a general item which i listed with both a start price and buy now option. I have gone to edit the listing, but it wont let me change the pricing. There is an existing offer on the listing so this may have something to do with it? I want to reduce the buy now price. Does anyone know why this isnt working for me? It does let me make changes to the start price (not the buy now) but when i select save changes it says 'the start price cannot currently be set'