Nedarim 43: Very Helpful Work-Arounds

A new mishnah: When one wants to borrow a cow (!), and the cow isn't available, which leads the would-be borrower to swear off the cow ever coming to plow the field. But when one who has sworn of benefit from another and is truly in need, the one who can't give CAN work around the prohibition and vow via indirect benefit to the other. But R. Yosi doesn't like the indirect giving as a work-around. Which makes the Gemara explore why he doesn't. Is it just timing? The way he conceives of hefker? The way he conceives of vows?