I think some people in this thread may have been a bit hard on you, considering you were not dishonest in any way. That being said, maybe you trying to make the excuse that the system didnt really fit his car well anyway might be a sign that next time you will think twice about agreeing to a trade you know is 1-sided with someone you call a 'friend'.
I hope you are both this happy with the trade 6 months from now. Cheers.