Xero have added a new feature giving you the ability to set a credit limit for a customer so you can keep track of what they owe you.
The credit limit can be set within the contact record, see the help notes here
You can also add a credit limit block to a customer to stop invoices from being sent if they have exceeded their credit limit, however this doesn’t apply when repeating invoices are automatically approved or approved for sending.
(this feature is only available if you have switched to the new invoicing)
To check a clients credit limit while generating an invoice, see the help notes here