Thank you for your question
My name is ***** ***** I shall do my best to help you.
You have no liability for any "invoice" and you should report this man to your local Trading Standards department immediately.
If the firm is connected to one of the trading federations you should also report the firm to them.
Please ask if you need further details
You are most welcome - and if he contacts you again call the police!
personally I would leave it - their approach is unprofessional at best