Partial lien release appraisal question.

[Thomas Brittingham]

So my principle residence sits on 4 acres and I have someone interested in exchanging some of my unused portion (1-1.5 acres) for a rental property. My home and land is still under a mortgage and therefore, I have to do a partial lien release. In the banks own words: " Fund's to lower the loan's principal balance may be required, even if there are no proceeds from this transaction. The funds required may be more than the proceeds of this transaction." 

My question is this: What if my house minus the acre or so of land appraises HIGHER than the principal balance owed on the note?