If you receive a warning that the order balance will be negative, then a settings has been enabled to prevent additional invoices being recorded against an order once the order is fully matched. The setting is in Business > Settings > Validations that enabled you to stop adding more invoices once the order is allocated. 

If you get any errors relating to this just turn off the “close purchase order for invoice matching when order is fully matched” setting and try again. You can then turn the setting back on afterwards.