The sugya of Heileich is one of the most famous in Shas.
Equally famous is the difficult Rashi. The conventional understanding is the הילך means הי לך -here, says the borrower - presumably at hand - is the money that I owe you. But Rashi says the money is not at hand, it is elsewhere in the possession of the borrower, who never actually expended the money:
How can one say that the money still belongs to the lender? As the Gra notes, the Ran has a similar explanation, but only in regard to a pikadon, which does in fact belong to the mafkid wherever it is.
I believe that the explanation of Rashi can be found in the ראב"ן:
explained by the Even Leshem b'Dvir:
Clearly, the Ra'avan reflects Rashi's understanding. Even when it comes to a loan, the money has the status of a pikadon until actually expended!