"The debt secured by the mortgage described above has been partially paid. Therefore, this discharge does not constitute a satisfaction of the debt. ... This discharge serves only to release the lien of the mortgage upon the property described therein."

Sounds like the initial mortgage was a blanket mortgage covering more than one property (think subdivision with blanket mortgage covering multiple lots? Or large farm acreage broken down into smaller lots and sold off one at a time) - this release only releases the one specific lot/parcel for a smaller specific payoff amount that doesn't pay off the entire loan - it does not discharge the debt as to the balance or release the lien as to the remaining properties used to secure the mortgage in the first place.