Generally, to claim a refund, you must file Form 1040-X within 3 years after the date you filed your original return or within 2 years after the date you paid the tax, whichever is later. Returns filed before the due date (without regard to extensions) are considered filed on the due date, and withholding is deemed to be tax paid on the due date.
Having said that, you may now need to recognize the gain or loss on sale of property. For the buyer to have been able to refinance means that transfer of property happened at some point. When that happens will be the date you will use for sale of property. If that happened in 2019, then you will recognize the sale in 2019.
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