Is it possible to deduct $ only when service completed?

I’m working with a lawyer on a site. He needs (ideally) to charge clients, but not actually complete the credit card deduction until he provides his paid service. Legally, as a lawyer, he is not really able to charge except when a service is delivered. But he needs some way of:

1. Ensuring clients are serious and don’t waste his time.
2. Possibly refusing an approach for legal advice if it’s not in his area of specialisation. In this instance, the transaction would be cancelled. But to satisfy condition #1, upon approaching him for his service clients need to give their credit card details and a payment deducted, or held in a kind of limbo state.

Rather like commonly happens at hotels. They put a certain charge on your credit card as a kind of security deposit. But it’s not deducted as such. It’s more “earmarked”.

I’m not sure if this can be done with any payment gateways. He is located in the US.

Has anyone faced this before and found a way to do it?

Any and all advice appreciated.

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