[Citroënbender]

You should look to this old example, for a hint:

A doctor and a lawyer were talking at a party.

The conversation was constantly interrupted by people describing their ailments and asking the doctor for free medical advice.

After an hour of this, the exasperated doctor asked the lawyer, "What do you do to stop people from asking you for legal advice when you're out of the office?"

"I give it to them," replied the lawyer, "and then I send them a bill."

The doctor was shocked, but agreed to give it a try.

The next day, still feeling slightly guilty, the doctor prepared and sent off the bills.

When he went to his emails later on, there was an invoice from the lawyer.