Under which circumstances is it acceptable for an employer to withhold pay from an employee?

Honestly they do expect that you actually SELL the product, meaning in their eyes you were not doing your JOB. Just because you claim to have made 20 appointments, does not prove you did anything worthwhile if you did not sell anything, after all I will assume the JOB was SALES. You might want to read over the paperwork explaining that more than likely you do not get PAID unless you actually complete a sale.