I'm not sure that this is the case when talking about such low amounts.
Taking hints from a similar "agent with percentage", think real estate agents. Unless an agent is working with pretty high level homes, the 3% they are getting is not significantly effected by a something like 20k up or down on a $300k house. It's an extra $600, but often their time is better spent getting the deal closed as quickly as possible for as many people as possible. If that $20k for the client takes an extra few days the haggle, the agent could have been working with another client on another $300k house deal.
In both cases, the thing that throws the agent and client out of alignment is the fact that the deals come through very infrequently, thus from the agent's perspective, they need to keep clients hustled through the door in order to be able to keep their own cash flow going, and working extra hard for a minimal gain isn't going to generally effect future gains. Most engineers would just use the same agent regardless unless the agent was spectacularly bad.
(and there are way more rules for real estate agents than there would be for a hypothetical engineer's agent)