The way I suggest understanding the Gemara in Bava Metzia on page three is that the Gemara is going like Sumchos, but can also be understood as going according to the sages but with difficulty. [Explaining this gemara like Sumchos is how I answered the question on the Rambam who brings it but without the answer of the gemara.] The Riva says the one guarding an object is not counted as if they are holding it. Thus, we can understand our Mishna as intending to say that two people holding onto a garment that they found is a case where there is no derara demomona. Derara demomona means according to Tosphot and the Riva that they lose no money. That means since they both found a lost garment, the case is not derara demomona since even if one would not recieve anything he not be losing anything. He simply would not be gaining anything. Since it is no Derara Demomona and the gemara in Bava Batra holds where there is no derara demomona, Sumchos agrees with the sages, and so he does not say his usual law of ''they divide with no oath.'' But then the gemara asks on our Mishna from the case of a third party that is holding an 200 for one person and a hundred for another and he does not recall to whom is what, and that is considered as not as holding in their place. It is rather a case where he is holding it for them but not as if they are holding it. and we understand the reason the third hundred stays in its place this is a case of derara demomonaa since the one who gave 200 to guard is losing some of the money that he gave to the third party to guard and since he loses a hundred. It is a case of derara demomona. So then why do they not divide? and why is there an oath? Answer: If they would divide, that would be a case where the verdict would not correspond to the objective fact that only one of them gave two hundred. [Thus, even though it is derara demomona, they still do not divide.]------------------------------------The way I suggest understanding the גמרא in בבא מציעא on דף ג' is that the גמרא is going like סומכוס, but can also be understood as going according to the sages but with difficulty. [Explaining this גמרא like סומכוס is how I answered the question on the רמב''ם who brings it but without the answer of the גמרא.] The ריב''א says the one guarding an object is not counted as if they are holding it. Thus, we can understand our משנה as intending to say that two people holding onto a garment that they found is a case where there is no דררא דממונא. NOW דררא דממונא means according to תוספות and the ריב''א that they lose no money. That means since they both found a lost garment, the case is not דררא דממונא since even if one would not receive anything, he not be losing anything. He simply would not be gaining anything. Since it is no דררא דממונא and the גמרא in בבא בתרא holds where there is no דררא דממונא, סומכוס agrees with the חכמים, and so he does not say his usual law of ''מחלקים בלי שבועה.'' But then the גמרא asks on our משנה from the case of a נפקד that is holding 200 for one person and a hundred for another, and he does not recall to whom is what, and that is considered as not as holding in their place. It is rather a case where he is holding it for them, but not as if they are holding it. And we understand the reason the third hundred stays in its place this is a case of דררא דממונא since the one who gave 200 to guard is losing some of the money that he gave to the third party to guard . Since he loses a hundred, It is a case of דררא דממונא. So then why do they not divide? and why is there an oath? Answer: If they would divide, that would be a case where the verdict would not correspond to the objective fact that only one of them gave two hundred. [Thus, even though it is דררא דממונא, they still do not divide.]