My Sold Bike Name Remains Unchanged

Dear All, I sold my R15 V2 in 2017 to a mutual person, at that time I didn't got any details or proof of selling my bike. Recently when I was checking if there is any challans for my current bike and i also casually check challans for R15 as well, there are multiple challan and he has paid as well but one challan is pending for 2 months. He has changed bike colour, i saw this in the digital copy of challan where photographs of him driving the bike was captured by traffic cam. I asked him to change the name while selling itself but he has not changed till now. I'm worried if he commits anything wrong and I will get into problem. What I can do now? What are the documents required if I need to file a complaint. I have only copy of RC book