How can negotiate in a lucrative way my new commission-based salary?

I started a perm Sales job 6 months ago paid £17k. If I get a new order or a new customer, I get £65. I accepted this job because I needed money and I only do what I am told to do, no more as I used to work as a Marketing Manager before. I don’t want them to take advantage of me. Anyway, I spoke with the MD about the possibility of getting commissions instead. He said that I would need to speak with my manager and he may decrease my salary. My manager is jealous and undermining because he fears I can be better than him. He knows that I am ambitious. Knowing that in this business, an excellent Sales person can get a max of 6 orders a year, order between £20k and £100k. I am sure that in May I might make 2 orders. How can negotiate smartly and lucratively my new commission-based salary? Please help. L