There is an argument between almost all other Rishonim and the Rambam concerning the question if one grabs a doubt.  The Rashba is usually in the side of the other rishonim in this kind of argument but in this case he brings a case for the Rambam and the Gra also.
The issue comes from a Gemara in Bava Metzia . R. Hanania said to Raba: there is a braita that supports your view that says the doubts go into the stall to be taken for the tithe. So if a kohen would grab a doubt and we would not take it out of his hand then one would be taking tithe by the money of a kohen.
The Rashba says that  that is only in the case of the first born of an ass which is at first owned by the Israeli. That is the status of the first owner causes us to say we would take it out of the hand of the kohen.

This comes up in the Avi Ezri of Rav Shach in laws of acquisition 3 law 9. I mean to go into this in a little more detail.