Enable a fixed partial payment

As an admin l would like to set a fixed partial payment for all automatically generated invoice so that l will not have to manually go and edit the invoices inorder to add a minimum partial payment for each client.
The system shall have 2 new option
To set a fixed amount
To set a percentage amount.
The system shall have a save button to enable the new partial amount.
Admin shall be able to change the amount or percentage amount at anytime.