If you're looking for a starting point, you can start your letter off something like this:
Dear [name]:And then go on to describe what you don't like about his actions. Try to stick to short, factual sentences that explain what he has done. Then, in the conclusion, write something like:
I am writing to express my formal dissatisfaction with [business agent's name]. He has been...
I sincerely hope that we can work out our differences and...
... Thank you very much.