R' Chiya says that included in the case of modeh b'miktzas where the person partially admits and partially denies, is also a case of "hei'lech". Meaning, Reuven claims 100, Shimon responds by denying 50 and admitting on the other 50 saying "hei'lech". R' Chiya holds that shimon is required to take a shavua on the 50 that he denies. R' Sheishes disagrees and holds that hei'lech is patur, because the 50 that he is admitting on is as if it has already been returned to the lender and the entire claim is only on the 50 that is being denied, so it is a case of kofer hakol which does not require a shavuah.
What is "hei'lech"?
Rashi writes - והילך - לא הוצאתים והן שלך בכל מקום שהם
Rashi implies that in order to qualify as hei'lech in a case of a loan, the borrower must say that he didn't spend the money, but if he would have spent the money and is presenting the lender with other money, that would not qualify as hei'lech. The hagahos ahsri understands rashi exactly like this, and therefore holds that if the borrower spent the money and is now replacing it by returning other money, even though we pasken like r' sheishes that hei'lech is patur, the borrower would have to swear because this isn't a case of hei'lech. However, the Bach on the Rosh says that rashi is not coming to define hei'lech, rather he is coming to explain r' chiya who says that even by hei'lech the borrower must swear. Rashi is coming to say that even if the money has never been spent and it is a supercharged hei'ech, r' chiya would still hold that the borrower must swear. But it is entirely possible that Rashi would hold that according to r' sheishes that hei'lech is patur, it would even be patur if the original money was spent so long as now the borrower is presenting the lender with other money in its place.
The Gr"a quotes from the Ran that a case of a loan is always considered as if the money has been spent since it is given to spend, and therefore a loan never qualifies as hei'lech. The only situation of hei'lech is when one gives another something to watch, and then presents him with only part of it at the time he returns it and denies the other part of it. Based on these rishonim, in a case of a loan, even if it hasn't yet been spent the borrower would have to swear.