How to add discount in percent?

How to add discount in percent?

Hello, I created a formular that will copy a value (from a custom dropdown with the name "discount in percent"… the values are , 20, 30 and 50 etc.) to the Invoices module to the discount field of the subform. I used the following formula which works, well (sort of). It adds the discount in USD not in Percent. How can I do this? 

The current formula looks like this: 
  2. mp = Map();
  3. mp.put("Discount",discountId);
  4. update = zoho.crm.updateRecord("Invoices",invId.toLong(),mp);
  5. info mp;
  6. info update;

Thanks for help.