Update on Recruitment Agreement - for billing on higher side if replacement candidate is paid higher salary

Hi Guys, Can anyone guide me on how to prepare an agreement copy where candidates leave an organization within 3 months and we have to give free replacement but if earlier candidates use to get less salary and new candidates are paid more salary than the earlier one. Can we raise the bill for the higher side or can I put in my clause that we shall raise the bill whichever is on the higher side?
Awaiting your advice.

Where it is an agreement, both the parties have to agree. Will it be agreeable to the hiring party to pay at the higher rate?. At any rate you can make this proposal and if it is acceptable to the other party then it is very good for you.