@KalRe 

You've been receiving bad advice from QB employees.  @JaeAnnC's response of "The negative amount indicates that the balance has been paid and a refund was issued. You may leave it as is if your customer wants to apply this credit to future invoices." is contradictory and incorrect.

To answer your questions:

1) You don't need to create a credit memo first, you can just create a refund receipt.  A credit memo is issued when the credit will be applied to a future invoice.  A refund receipt is best when you want to issue a refund.

2) If you have -$100, then the customer has a $100 credit balance that can be applied to future invoices.  Issuing a refund receipt does not create a negative balance.  It doesn't affect the customer's A/R balance at all.  So, if you are showing -$100, you issued a credit memo or some other entry that credited A/R $100.  If you issued the $100 refund receipt, you can delete the entry causing the -$100 balance.