If one does a favor or service for another without previously
arranging to be paid, he may still demand payment for the service
and the beneficiary of the service may not claim that it was done
for free because no arrangement had been made. This is true
whether the provider of the service is a professional or not, and is
particularly applicable to situations where the beneficiary was
looking for someone or was interested in having the service
provided. (Rema î “çå 264:4)

Adopted from the shiurim of Ha Rav Leible Sternberg.