Reverse charge : service provider has charged gst on bill

Service provider falling under reverse charge ( Sponsorship/GTA/Recovery Agent) has charged GST on their bill. What should the service recipient do ?

a) Pay bill amount including GST and abstain from paying GST under reverse charge?

or

b) Pay bill amount excluding GST and pay GST under reverse charge instead?

Under GST laws is the liability of paying GST under reverse charge absolute?