Under Reverse Charge Mechanism, the Liability of Service Receiver is completely different and independent of the liability of the service provider.
What the service provider has done, is of no relevance. The service provider may not charge any tax in the inovoice (by mistake or when claiming basic SSI exemption of Rs. 10 Lacs) or as in your case, if he charge 100% service tax, the service receiver must lawfully and fully discharge his liability of 50% of service tax. However, your liability arises only after you make payment to the service provider within 6 months of the date of invoice, otherwise the date of invoice. (Rule 7)
He cann't issue any Debit Note as suggested by Mr. Sumit, as he is paying his own liability and on his own behalf. However, if the provider has charged tax @ 100%, he can lawfully deduct 50% tax from the invoice while making payment as he is liable to pay tax as per actuals and not any additional tax.
Regards,
tax2001-it @ yahoo.com