Audi - heavy service charges

we have given our audi car for service and they made a bill of ****** just for the AC compressor change. we bargained and finally were able to reduce up to ***** and closed the deal,whereas the market value of the original audi ac compressor is only *****. after the car was returned back to us the battrey was dead and car not started within a week. we again called them and they said they will replace the battrey for free of cost over the phone , but later we received a mail of rs ***** as a charge for the new battrey replacement . we do not understand what to do now.