The idea is to integrate the 500 THB diligence allowance (variable based on no sick leave in the month) in the fix monthly salary. This can be triggered once the salary review proposal has been approved or can we trigger separately from salary review?
Regarding how to do it, I am not sure of from a HR point of view what is the best solution. We usually write an amendment to the contact to specify the change on the contract condition, such as transport allowance. I will send you shortly the template (my external hard drive is at home)