I want to fire my employee for declining a promotion
I am confused. Isn't there someone else who can take the job?
There might be someone that could do it better, but I wanted a specific employee to take it
It sounds like this must be a good employee, and it would be wrong to fire them for not taking a promotion. Just consider offering it to the next person in line, and leave this employee in their current position. Firing them for such a silly reason would cause a lot of undeserved stress in their life.
I don't want to lose the good employee that I have, but I am not going to let them get the better of me.
Firing the person that you want to promote doesn't make alot of sense because then you lose them forever. Consider that the person may want the promotion in the future when they are ready for it. Don't shoot yourself in the foot by firing them.