Have you been making the $500 payments every year since you sold the property ?
If so that was soooo wrong and you need to correct this error now. The easiest way is to just say you sold in 2020 so you pay back the balance due now on the 2020 return. The correct thing would be to amend the 2014 return and take credit for all the other payments however this is a pain at best and will not accomplish anything different in the end.
