Open Question: audit letter from department of taxation asking how much I sold my car for ?

I just got a letter from the department of taxation asking me how much I sold my car for. when I sold my car 6 months ago I notarized the title and gave it blank to the buyer. now the department of taxation asking me how much I sold it for and a bunch of other questions. I sold the car for 6000 the guy apparently put 1000 in the title so he wont pay tax on it. I do have a letter that says the car needs a transmission and engine work that will cost about 4000. so if I tell them 5000 he will get in trouble and if I say 1000 than I'm not being honest with them. what should I do ??? I'm I in trouble ?