****med by buyer on pay pal

was selling my laptop on internet got a buyer he then payed in the agreed amount into Paypal,Paypal then confirmed the amount but to release the money they need the tracking number,i pay R890,00 for the postage and the laptop have been sent now the customer want me to pay a further R4000.00 FOR THE CUSTOMS duty it have been sent international.paypal told me the funds would be released 48 hours this is already 4days and no payment,i never agreed to pay export duty and would never have sent the item without been getting pay,but paypal insist in the tracking number.so now i am out of money and out of my laptop