How to record minimum concession amount

Dear All,

I want to record minimum concession amount in variable rent. how can i record it in variable rent ?

For Example:

Min Concession Amount : 5000

Case 1

Sale turnover = 40000

Rate 10%

Variable rent for this case is 4000, This amount is less than 5000 so customer need to pay 5000 because minimum concession amount is 5000

Case 2

Sale turnover = 60000

Rate 10%

Variable rent for this case  is 6000, Now this amount is greater than 5000 so customer need to pay 6000.

How can i achieve both cases in variable rent ?

Thanks