Poor Workmanship

I paid about R5000 for a pool cover. Initially a guy called Emil came to check the pool and indicated that the cover will take the shape of the pool. I had to pay an extra R3365 for that. He sent his guys for the customization of the pool cover as it is not square nor rectangle. The first time they came for fitting, I had to sent them packing, as the cover did not taking the shape of the pool as agreed. Yesterday, they fitted the cover I was busy working, today I checked the cover and realize that there are some edges that are not protected from further tear and this guy wants me to pay additional R450 to fix such when it's not my mistake.