IDEA: Do not try to negotiate price after the vendor’s work is done. Be ethical; if the work is acceptable, pay him the agreed upon price.

Discuss and agree upon a price to do a job or make a product or provide a service with your vendor, then put the agreement in writing and issue him a purchase order or at least your offer in writing with a signature. Many vendors will start work without the paperwork trusting the company.  Once the work is complete, do not try to go back and negotiate the price with the vendor once he has completed his part of the agreement. You will quickly ruin your credibility with everyone.