The creditor does not view the property pledged as security as a commodity that can be exchanged for cash in the market to pay off the debt. He is not interested in the cash value of the security but in the security as property for his own use. This has two important implications. On the one hand, the borrower cannot make further loans on the security after pledging it to one creditor. On the other, if there is a difference between the market value of the security and the amount of the loan, the borrower does not have a right to the excess, and the creditor cannot demand the payment of any deficit.
In more general terms, it means that the creditor does not view the security as a commodity,only as property capable of being transferred to his ownership"...M.I.Finley