How to add discount approval proccess for Estimate in CRM

How to add discount approval proccess for Estimate in CRM

Dear All, 
After we do integration of Zoho CRM and Book , the estimate is controlled by Zoho Book.
We would like to have approval if the staff makes discount in Estimate : 
such as : 5-10% discount : need team leader to approve 
                 >10% disount : need boss to apporve 
In Zoho CRM , there is approval process, but there is no control for Estimate Modul 
In Zoho Book , there is Sales Transcation approval , but no classification , and we dont require approval for all transactions.

So is there any suggestion on this ?