ribbis question

I know I'm falling a little behind on divrei torah here, and I will try to catch up.  in the meantime, I'm learning eizehu neshech with another guy here at weizmann, and there is something that bothers me significantly.

In general, the rule is that masneh adam al mah shekasuv batorah bidavar shebimamon- a person has the right to waive issurei torah that involve one's own money.  for example, although the torah assurs collecting a loan after the shemittah year has passed, if both the borrower and lender agreed at the time of the loan that this loan would extend even past shemittah, their condition works - the lender has the right to collect the loan even after shemittah.

a notable exception to this principle is ribbis - even if both parties are willing to initiate a loan with interest, it is prohibited to do so.  why?

I always thought the reason was (and I would swear that I have seen this brought down as the reason) that unlike other monetary issurim, which only address one party, by ribbis, the torah gives a prohibition to both the malveh and loveh.  In other words, by shemittah, the torah doesnt put a blanket assur on collecting a loan after shemittah - by addressing the issur only to the lender, the torah is essentially granting a zechus to the borrower.  Since it is his zechus, he has the right to waive it; if both parties agree, a loan may be initiated that extends past shemittah.  Ribbis, in contrast, has an issur directed specifically to the loveh - this teaches us that even if he would be willing to forgo the zechus to a loan without ribbis, he is not entitled to do so.

But the guy I was learning with argued out that you dont need to come on to the above to see why ribbis is different.  Even if the torah only assured ribbis to the malveh, we would still know that the issur is different from other monetary issurim and cannot be waived.  Why?  Because if ribbis could be waived, then you would never have a case where ribbis is assur.  If the loveh agreed, the issur would be waived, and if he didnt agree originally, then it wouldnt be ribbis dioraysa (ribbis deoraysa must be agreed to at the time of the original loan).  By shemittah, even though the issur can be waived we have a case where it applies - where money was lent without any special preconditions.  For ribbis, we have no such case.

So my question is, whose right?  I am convinced that I have seen sources that explain the rationale for ribbis being different the way I explained it, but if so, how do you deal with explanation #2 (we have to find some case where ribbis deoraysa would exist even if the issur could be waived).  Any mareh mekomos/ hearos much much appreciated.  Thanks!