Do you have sales Invoices turned on or are you simply receiving money and entering the customer account?
Typically you would use sales invoices to invoice your customer: Dr Receivables / Cr Sales Revenue
(You could alternatively do this with a journal entry)
Then receive cash from the customer: Dr Cash (Bank) / Cr Receivables
Then buy from suppliers using Purchase Invoices: Dr expense or Inventory etc. / Cr Accounts Payable
Then pay cash to your supplier: Dr Accounts Payable / Cr Cash (Bank)
My guess is that you are using receive money to receive money from your customer before recording the fact that they owe you money. If you get money from a customer who doesn’t owe you anything (i.e. there is no open receivable invoice), then the customer has a customer credit, which is indeed a liability to you.
I’m guessing the situation here based on what you are describing, but could it be you are simply missing the step of recording the receivable (and your revenue) before receiving the cash to pay that receivable?