Just as landlady's are not in the habit of giving their beer away for free, neither do plumbers, electricians, singers, or dee-jay's, provide their services without charge. Perhaps they may occasionally, most likely for friends or charitable organisations, but ordinarily, they charge for their services.
Small claims are decided on the balance of probability. In other words, whose story, on balance, is likely to be the most truthful? I'm not daft, you're not daft, and neither is the Sheriff or Judge/Magistrate.
It could be that the karaoke was provided free of charge, but common sense and objectivity tells us that it is more likely it would have been chargeable. Especially in view of the known facts that another entertainer had cancelled at short notice, and that the singer had been paid for a performance within the same premises. The circumstances made it unreasonable to expect the contracting parties to formalise the agreement in writing, which in any case, is not necessary, though is preferable for evidential purposes.
In those minority of occasions when you don't formalise your agreements, Reverand, do you not think you'd be able to win a small claims hearing if the client failed to pay what you had verbally agreed to?