Variable Rent with % sales calculation

Dear Team,

We have a requirement to implement a billing condition for one lease based on the total sales.  Please find below the scenario with an example.

 

There is a lease where we have fixed base rent eg 1000 BHD, which will be billed on quarterly advance.

But if quarterly sales of the customer are more than 20,000
BHD then we need to bill 10 % of the total sales ie the invoice amount should
be 2000 BHD for the respective quarter. If customer sales are less than 20,000
then we will bill only the minimum base rent ie 1000 BHD.

Our current process, we will bill the customer 1000 BHD