Paypal — Making PayPal account negative

PayPal has made my account negative by USD 725. It says that buer has made charge back. I have spoken to buyer and has a letter as well as account statement that there was no chargeback. Still, PayPal is not listening and coming up with an excuse every time. I have received cheque from PayPal o[censored]SD 725 (converted into INR) but the PayPal has made my account negative. I am not sure what is happening. I have raised compalin with PayPal but there I am not sure what is happening. Plus, they have threatened me that they will close my paypal account in case I do not pay them. Please help. I have necessary proof that buyer and his bank has sent money via paypal and then paypal has sent me the cheque with INR conversion.