Dear Prasanna,
The heading of your post should have been "Conditional Salary Increment" and not "Conditional Offer Letter."
You have asked for help in drafting the letter. However, I request you to draft the letter on your own and upload it on this forum. Some seniors, like me, will correct it. I say so because we, the members of this forum, are outsiders and we do not know about the length of the service of the employee, the nature of the job done, what the percentage of the hike will be, and above all, what condition you would like to link to the salary hike.
While I do not know the internal dynamics of your company, with my experience, I can say that either it will be locking the employee's tenure for a certain number of years or enlarging the scope of the work.
Anyway, write the details of your requirement.
Thanks,
Dinesh Divekar