How to link Accounts Receivable with Customer?

There are a couple points @alasdair did not mention in this or his answer to your other question:

  1. Accounts receivable and Accounts payable require that the customers or suppliers, as the case may be, be created first.

  2. In Manager, entries are not made directly in those accounts. Instead, when creating invoices, the customer/supplier is named and transactions are assigned to income or expense accounts. Manager automatically allocates to the appropriate subaccount under Accounts receivable or Accounts payable.

  3. Neither of those accounts should be used unless you are buying or selling on credit, in other words, if you will owe your supplier money or your customer will owe you money for some period of time. If, in a retail situation, the customer pays immediately at time of purchase, you should just use Receive Money in a bank or cash account tab. Frequently, a retail business can total transactions for the day and make a single entry. There is usually no reason to create a customer for a retail transaction. Your suppliers are probably different, so you will likely wish to create them in Manager. Even so, if you pay immediately upon delivery of goods, you can just use Spend Money.