" I owe a significant amount."
You don't want to send a significant amount if you don't have to.
Change the amount on line 18 to ZERO.
Explain what you did and why in Part III. (i.e. received nothing).
If you are using TurboTax Online you will have to get the Form 1040-X in fillable PDF from IRS.gov and copy all the other lines above, then redo the arithmetic below, since TurboTax Online makes it difficult to change Line 18.
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