Thanks for joining the Community, SAMA2018.
 

You can follow AlexV's detailed steps to apply a credit memo to an invoice without receiving any money from your customer.
 

If for some reason you continue receiving a negative amount, I'd recommend getting in touch with our Customer Care team. They'll be able to pull up your account in a secure environment, conduct further research, and create an investigation ticket if necessary.

They can be reached while you're signed in.

Here's how:

1. Use the Help (?) icon.
2. Click Contact Us.
3. Enter a description of your situation in the What can we help you with? field, then hit Let's talk.
   
4. Select Start messaging or Get a call.
   

Be sure to review their support hours so you'll know when agents are available.
 

I'll be here to help if there's any questions. Have a lovely day!