Override Installments on a Sales Order

Good day,

For our client we are in need of a way to override installments on a sales order. Currently there is an option to show the 'Override Installments' checkbox on invoices. This feature is needed on the sales order as well.

The business case:

The department that creates the sales orders is in contact with the customers. During negotiations they create contracts that decide what the invoice schedule will be and what the total amount to be invoices will be. After the negotiations are done and the contract is signed, a sales order for a particular period will be created. In practice this means that a sales order for an entire year will be created. Since the invoice terms and schedule is decided beforehand, this needs to be reflected on the sales order.