You can set a default credit limit for all your customers (To set default credit terms for all new customers). You can also set a credit limit for an individual customer (To set or change credit terms for a single customer).
To make these limits useful, you can set a preference in the Preferences window - Sales view to be warned before recording an invoice if the customer's credit limit is exceeded.
If you do both those things, set a credit limit for a customer and select the Credit Limit preference, when you enter a sale for the customer:
The alert message you receive informs you that the credit limit either has or will be exceeded and you can either continue with the sale or cancel it.
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