Thing is, in a market system you don't have to do your best work, you just have to be a better option than anyone else.

Imagine an employer has a job that they expect to take 40 hours per week, and they have two possible hires, Bob and Sally. Both Bob and Sally are willing to do the job for the same pay, but Sally looks a lot more competent, so the employer hires Sally under the agreement that she gets paid for at least 40 hours of work just as long as the job gets done, with overtime if it takes her longer. The reason the contract looks this way is because to the employer the job is valuable as a task, but to Sally it's valuable as an amount of time she's setting aside.

The next few weeks, the employer notices that Sally is getting the job done in only 20 hours and then doing her own thing. Now, the contract states that she gets paid for 40 hours, but it also defines the job. So the employer fires sally and hires Bob.

Turns out, Bob takes 40 hours to finish the job and costs the employer the same amount he was paying Sally.

What the Employer didn't realize is that Sally's bid was equal to Bob's because in that market, a competition to complete a task for a set amount of pay, Sally's results were equal to Bob's but her time was more valuable. The employer could have contracted her to only work 20 hours, but then she would have charged twice as much per hour, because that's what the task was valued at. If the employer doubled her workload, he'd be getting twice as much value out of Sally as the market determined she was worth, so she was never going to agree to that Contract.