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I'll help you with attaching a sales receipt transaction to your existing customer, Danfrey.
You can create a sales receipt for your existing customer from the +New icon or the Customers window.
On the other hand, here's how you can add attachments to your customer and his or her transactions: Attachments in QuickBooks Online.
Aside from that, you can review the topics from this link for more resources while handling your sales and other customer entries: Topics about your company's income and customers.
If there's anything else you need help with attaching sales receipts to your customers, let me know by adding a comment below. I'm more than happy to help. Keep safe!