@Emancipare 

"...so it seems the sales receipt is the one attributed to the invoice, and it's the payment that's dangling out there. The payment in Client A's transaction list says Closed whereas the Sales Receipt says Paid, as seen in the screen shot. Void or delete one or the other?"

That's not quite correct.  If the payment is showing in undeposited funds (UF), and you know that's a duplicate transaction, then delete both invoice 126 and the payment.  Sales receipts record the sale and payment in one transaction - there is no separate payment.  Invoices are a sale in one transaction and the payment is a second transaction that pays the invoice and closes it out.  That is why you need to delete both.