A negative biopsy is good. It could be from rubbing on the silver filling, so see what happens once corrected. Is the white patch able to be scraped away or not, raised, is it more of an ulcer? There are many types of oral conditions, medications, substances that can be the cause, some able to be scraped off, and more of an infection, recurrent conditions that come and go, ulcers. There also could be co-infections or diseases in any situation, and not always cancer, so although one condition is identified they may not look further once one cause is found. Maybe see an ENT, ear nose throat doctor, that deals with this more often to help identify, your primary care doctor or infectious disease doctor to do blood work, cultures tests for infections, diseases. Good luck.