Unless paying them that amount means that they will burn out on the job quicker and move on... Opening up the possibility that they can't find an adequate replacement.
This is also more likely if the person is a poor negotiator as they will be more likely to just find a new job when they get unhappy rather than ask for a pay increase.
I don't see a reasonable alternative, short of a fixed salary. Fixed salaries are a win for poor negotiators, but don't give companies a chance to sweeten the pot to retain someone more expensive than their fixed offer.
This is also more likely if the person is a poor negotiator as they will be more likely to just find a new job when they get unhappy rather than ask for a pay increase.