You can enter cash receipts. Read the Guide at:
https://forum.manager.io/t/cash-sales-selling-without-sales-invoices/7321
And you can make credit sales with a sales invoice:
https://forum.manager.io/t/creating-sales-invoices/7178
Customers are defined as described in the Guide below:
https://forum.manager.io/t/creating-and-using-customers/7018
Although not mentioned specifically in the Guide above, one of the parameters when creating customers is credit limit:
