Helene,

It sounds like the same thing I have on my land contract with my mother-in-law. It essentially keeps anyone with an interest in the property from further encumbering the property which would result in a change of lien priority.
The neg. pledge agreement that you're talking about is usually just a clause in the body of the instrument but can sometimes be a whole new instrument especially in your case of a commercial transaction.

I'm more interested in your promissory notes. Sounds like you have alot of digging to do on this one. I hope you are charging accordingly for the search and any title defects you have to correct.