The Mishneh L'Melech (hilchos malveh 4:2) quotes a Rashba who is medayek from our gemara that the issur of avak ribbis is only an issur d'rabonon on the lender, not on the borrower. The gemara says that R' Ilish would never have signed a contract of Iska that would consider him a borrower on half, because the work that he would be doing for the lender would be considered ribbis and "rav ilish would not feed issur to the lender". The implication is that the only issur on the borrower of avak ribbis is an issur lifnei iver that you are causing the lender to violate an issur d'rabonon of taking ribbis, but there isn't any specific issur d'rabonon on the borrower of avak ribbis. This is also paskened in the Rama (y.d. 160) who quotes this from the Rosh, that the borrower of avak ribbis is only in violation of lifnei iver.
The Chavos Da'as points out that perhaps there is a specific issur d'rabonon on the borrower to pay avak ribbis, but in this case r' ilish died before he actually paid it. Had R' Ilish stayed alive, perhaps he wouldn't have paid the avak ribbis because it is an issur. The gemara therefore can only make its point by saying that R' ilish wouldn't have entered into an agreement by signing a contract that would state an obligation to "feed issur to the lender" because this is also lifnei iver.