I think the Rambam does not hold with the law of R. Isaac. The statement of R. Isaac is this: a person that lends money to another, and receives a (mashkon) guarantee for it acquires the guarantee. My reasoning is this. The Gemara in Kidushin page 8 says if one sanctifies [marries] a woman by means of a loan, and gives her a guarantee (mashkon) to hold until he gives her the loan, she is not married by this because there is no loan, and there is no mashkon. And this is the law. If r Isaac would be correct, then she would be married. Just to add a bit of support to my opinion I would like to site the Shach on the Shulchan Aruch Choshen Mishpat, chapter 72 halacha 2 where he shows that if R Isaac is correct, then the receiver of the guarantee is considered a borrower, and the Gra in paragraph 11 agrees with that reasoning. But we know the Rambam holds the lender who receives the guarantee is considered a paid guard. Furthermore, the Rambam does not make any distinction between if one explains or does not explain that the guarantee is for the loan as we see in Laws of Loans chapter 13 law 4. I might mention that this all occurred to me when I saw the Ri Migash mention that Gemara in Kidushin. He (the Ri Migash) does make that distinction between if one explains or does not as the Gemara in Bava Metzia says, however the Rambam makes no such distinction. Now I might also mention that I realize that this opinion of the Shach that the lender is a borrower is the opinion of Rashi, and that most other Rishonim seem to disagree with it. However, the Gra does agree with the reasoning of the Shach.