Negative Grand Total for Quotation/Sales Order

In our business, it is sometimes necessary to create a quotation that credits products that were sold earlier and charge for new ones. This will sometimes cause a negative grand total or refund to the client. For some reason, ERPNext will not let me save a quotation that does this.

Is there a way I can allow quotations and sales orders to be saved and submitted even though the grand total is less than zero?