I would like to suggest that what Rav and Shmuel said about the block of metal of R. Aba exactly like the gemara in bava metzia page 100. They said he keeps it and the gemara in bava metzia first asks, "let's see in whose domain it is." From that we see we look first at where the object is now and only if that doe not work then we look at the first owner.And that does not disagree with the gemara in Nida about the חזקה מעיקרא being stronger because the חזקה of money is different than a חזקה about איסור והיתר .So then why does the Rashbam say the reason of Rav and Shmuel is because of a migo? Answer: The Rashbam is posek like Sumchos. Now this depends on the version in Bava Metzia. If the right version is הא מני סומכוס then sumchos agrees that we look at the domain where the object is now. But if the version is אלא הא מני סומכוס then sumhos says even when there is a present domain still ממון המוטל בספק חולקים then he would need a migo. The trouble with this is the Rashbam holds the first version that I mentioned is right. Therefore we need to go to Nida page 2 and the Tosphot there where there is an opinion חזקה מעיקרא וחזקת השתא are equal. So here we have מרא קמא and חזקת רשות which should be equal except the migo turns the weight of evidence towards the person that grabbed the נסכא
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