When I used to install cameras, I got into the habit of a labor down payment and it would be a fixed price.
I would never negotiate with Indians, and I would always stipulate in the down payment that arguing the price at any point meant declining the contract and the down payment would be forfeited. The second you give them an inch or even think about giving them an inch they interpret it as guaranteed discount.
Then they would be billed for materials afterwards, which was also non negotiable. The contract also stipulated that I would be entitled to file a lien within 30 days of nonpayment and I was entitled to put a remote turn off in the system that would disable the system until payment.
They NEVER read the contracts. They threatened to sue, call police, and sent death threats. They always thought they were going to screw me and not pay because upfront costs were 25-40% of the whole price. I did 30-40 jobs for Indians over a 2 year period and all of them except for a couple ended up in legal fights. I forced foreclosure on a few of the homes and tag teamed with other contractors they tried to screw over.