I receipt a refundable payment from a customer, which is unearned revenue for me. How should I send a receipt to the customer as he/she request?

Hi useraccounting,

Thanks for joining us! With QuickBooks Online, it's simple and easy to record your transactions so you can keep your books accurate. Sending receipts to your customers is important to provide proof of their purchases or returns. I'll be happy to assist you with this. 

To send a refund receipt to your customer via QuickBooks Online, you'll first need to record it in the system. Here's how.

1. Select + New then Refund receipt.
2. From the Customer ▼ dropdown, choose the customer you want to refund.
3. Select the Refund From ▼ dropdown, then select the bank you deposited the payment for the invoice too.
4. Add all products or services the customer returned in the Product or service column.
5. Make sure to fill the quantity, rate, amount, tax, and other fields accordingly, then select Save and send from the drop-down ▼ beside Save and close.
6. Click on Send and close.

And voila! To learn more about refund receipts in the program, check out this article: Record a customer refund in QuickBooks Online

I hope this helps! You're always welcome to stop by if you have other questions.