Abaye seems to hold from his question on Rabbah that "bva lichov bdmeihen" is no good even when the borrower has the product he is transfrering the debt onto. why isn't the case Abaye say's our mishnah is dealing with a case of Bva Lichov, he is transferring 30 debt onto wine?
Abaye's question on Rava was as follows: If you are learning that the Isur in the Mishnah is "Ba Lachov b'Demaihem" that one is not allowed to be Posek on the Sha'ar if the buyer is not giving cash, if so the Mishnah need not write "v'Yayin Ein Lo" -- the same problem would arise even if he had Yayin, since the buyer was not giving money at the time of the sale. Therefore, clearly "Ba Lachov b'Demaihem" is not the problem.
D. Zupnik