People accept pay offered to do a good job when they take a job. If they do an above average job once they are trained, they deserve a pay increase. If they don't do an above average job, then they shouldn't get a pay increase or their pay increase should be smaller. If they come back and insist they can do a better job for more pay, it means to me that they refuse to honor the committment to do a good job and are NOT doing their best job ... which indicates to me that they won't do it any better if they do get a pay increase. And if they refuse to do their best at the job - above average or below average - then they shouldn't have that job.