There is a machlokes Taz (170:3) and nekudas hakesef whether one can hire someone to be an cosigner on a loan where the cosigner has full responsibility as much as the borrower himself (areiv shluf dotz). The Chochmas Adam in his sefer Binas Adam (sha'ar mishpat tzedek 2) points out that this discussion took place before the ritva on baba metzia was accessible. However, now that it is accessible the answer is resolved. Rava says that Reuven may pay Shimon to go convince Levi to lend money to Reuven because reuven is only paying shimon for s'char amira. The ritva asks why does the gemara need to be matir based on this rationale, it should be mutar anyway based on the other rationale that the gemara suggests that the torah only forbids ribbis that is paid directly from the borrower to the lender. The ritva answer that we are speaking about a case where Levi refuses to lend money to reuven, but is willing to lend it to shimon who then goes and lends it to reuven. Even though it turns out that technically shimon is the one who lends to reuven and therefore when reuven pays him it is ribbis that is paid from the borrower to the lender, it is nonetheless permitted since it is only s'char amira. The binas adam learns from the ritva that it is only is a situation where reuven doesn't request of shimon to be an areiv shluf dotz, rather shimon decides on his own to borrow from levi and lend it to reuven. But, if reuven would be paying shimon to be such a high level areiv, it is as if he were hiring shimon to borrow from levi and then lend to him which is an issur of paying ribbis since it is no longer just s'char amira.