In modern terms this is called a lien. When someone borrows money with a Shtar he is giving the lender a right to collect from his property if he defaults on the loan. If he actually does default, then the lender's rights to the property precede those of the buyer, as long as the buyer bought the land after the lender's lien was made on the borrower's property (of course, every buyer must conduct due diligence and research before buying property to make sure that there is no lien on the property, or he should be ready to accept the risk of the property being confiscated). It is as if the buyer bought a property that already belonged to someone else. However, the Chachamim enacted that one can only collect from sold properties if the borrow does not have any money or unsold property left himself. If he does, then the lender must collect from them before he can collect from the sold properties.

D. Zupnik