iSupplier Portal - Configure Quantity field to be blank

Hello,

While creating an invoice in iSupplier portal, the quantity left to be invoiced (Availabile Quantity) automatically populates in the Quantity field.  Suppliers are submitting invoices withough checking this field, which is causing significant issues.  How can we make this field blank, forcing suppliers to manually enter the Quantity associated with the invoice they are creating?

Thank you!

Melissa