I have been thinking about a a certain problem in the Rambam Laws of Lender and Borrower perek/chapter 19 law 8 for a while but have not come to any clarity.

The basic subject is this: You have a lender and borrower. The borrower sells  a field to a buyer. Then the lender writs to that buyer "דין ודברים אין לי עמך" "You have no obligations to me." Then that buyer sells the field to a second buyer.  Then if the if borrower does not pay the debt, the lender can go to the second buyer and take the field as a repayment of the debt. [This is the regular law of שיעבוד that any property owned by a person at the time of a loan is obligated to pay that loan even if he sells that property or gives it away. The lender can always go after it if there is nothing owned by the borrower.]  

The problem that I have is that then the first buyer can go and collect the field from the lender because he will have lost the money that he had to pay to the second buyer  because of the loss of the field to the lender. I do not see why the first buyer can go and collect the field from the lender, for the lender did not collect the field from him. All he wrote was, "I will not collect the field from you," and he didn't..

I noticed this subject in the Avi Ezri and it is also brought in the Chidushei HaRambam of Rav Chaim of Brisk but how they explain this is not clear to me. I was hoping that thinking about this while at the sea shore would help but so far I have zero ideas.