Impacted versions: 
9.0x

Steps to reproduce:
1. Go to Accounting, create customer invoice and validate. Check the Invoice-Number (for example: INV/2016/0010)
2.  Go to Point of Sale and select New Session
3. Add any product or just close & confirm the session
4. Close the POS Session (Validate Closing & Post Entries)
5. Create new customer invoice and validate it. Check the Invoice-Number. 

Current behavior:
Gap between the first and the last invoice because of the closed POS-Session. 

Expected behavior:
No Gap between the two invoices like in Version 8. 

I've tested this behaviour on a 8 and a 9 runbot and my own system. There is no gap on 8 but on 9. I think this is not a feature but a bug. I can't find the missing sequence-numbers in any Journals or in the invoice-list. 

The only way to avoid this, is to select another Sales Journal in the POS-Configuration. But then you can't create an invoice from the POS. 

Is it possible to get the "lost" invoice-numbers somewhere? We are not allowed to have gaps in the invoice-numbering here in Germany. 

account_precision = self.env['decimal.precision'].precision_get('Account')
        
        if not float_is_zero(order['amount_return'], account_precision) and not float_is_zero(order['amount_paid'], account_precision):
            cur_amount_paid = 0
            payments_to_keep = []
            for payment in order.get('statement_ids'):
                if cur_amount_paid + payment[2]['amount'] > order['amount_total']:
                    payment[2]['amount'] = order['amount_total'] - cur_amount_paid
                    payments_to_keep.append(payment)
                    break
                cur_amount_paid += payment[2]['amount']
                payments_to_keep.append(payment)
            order['statement_ids'] = payments_to_keep
            order['amount_return'] = 0
    @api.model
    def _process_order(self, pos_order):