You cannot sell a portion of a property secured by a note without potentially triggering a "due on sale or transfer" clause.

I would suggest that you call their current Lender. I believe that they would need to refinance because their current loan is tied to the whole property, including the 1 acre that you want to purchase.

Your parents would have to sub-divide the property.

This would depend on the lender. Most lenders on rural owner-occupied property do not give any value on land over 5 acres. You need your parents mortgage documents.Then, I suggest you speak with a real estate attorney. A few buck, but it will be worth it. ... Happy funding, Rudi