Bava Metzia page 112. You have an artisan that fixed a vessel and asks 2 shekalim for his work. The owner of the vessel say the agreement was for payment of one shekel. One braita says you believe the owner and the other says you believe the artisan. Rav Nachman bar Izchak says the difference is when there are witnesses you believe the artisan because the owner has no migo (literally "he could have said...") to say לא היו דברים מעולם-I never saw you before. I wanted to say the reason Rav Nachman bar Izchak says this is that he can not make a difference between if the vessel is movable or not.  That is he hold even when it is movable the owner still has migo to say I do not know you because he might think the amount the artisan is asking is more than the actual worth of the vessel. I mentioned in my notes that I believe Rava disagrees with Rav Nachman bar Izchak and holds the difference is the case you believe the owner is when the vessel is not movable and so he has a migo.

[You can look at the notes but the simple and short of it is that Rav Nachman bar Izchak  was  going along with Rav and Shmuel that hold you believe a worker that says he was not paid only when there were witnesses that he was hired. Rava disagreed with Rav and Shmuel so I was suggesting that Rava also would disagree about the artisan and give a different answer that Rav Nachman]. That train of reasoning led me to find support to Rav Joseph Halevi that holds  a migo is causes one to be believed to say he does not have to pay money but does not absolve from an oath. The idea was there are versions of the  braita about the artisan and the owner in which you believe the owner. One is you believe him with an oath and the other is without an oath. The one with an oath would be like Rav Joseph HaLevi and the other like the Ran [Rabainu Nisim who holds a migo also lets one of the hook of taking an oath.]

I might add one thing I did not mention in my notes. This all occurred to me because I realized that almost every migo has something working against it. Just like in the Torah we have מודה במקצת הטענה נשבע because the Torah is thinking he wanted to deny everything but he would rather not because אין אדם מעיז פניו בפני בעל חובו. That is if he had denied everything he would have been beloved so why not believe him when he admits only a portion? Answer because a person does not have arrogance in from of someone that did  for him a favor. So the Torah put an oath on him. [I wrote that the argument between Rava and Rav Nachman bar Izchak is when you say a migo. I forgot what I meant when I wrote that. But today it occurred to me that the above idea is what i might have meant.]