Hi 8256342
Thank you for your post. For situations where a payment is made by a customer that you haven't invoiced yet, we do generally recommend creating a negative invoice like you mentioned which then creates a credit that you can then apply to the invoice at a later time.
The steps for this process can be found here: Creating customer returns.
You mentioned in your post that you are unable to do this, could you provide some further context into what is occurring? A screenshot of the transaction or any error messages that you receive would be helpful.
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