The Tosephta (BM ch 3) says המוכר פרה לחבירו ונגנבה זה אומר ברשותך נגנבה וזה אומר ברשותך יחלוקו This is clearly like Sumchos ממון המוטל בספק חולקים. Rav Shach says clearly the argument is when the theft occurred before or after the קנין. This can not be they are arguing in whose domain the animal as when it was stolen because that would not be דררא דממונא. we need they should themselves be in doubt for there to be דררא דממונא
But then it should be a simple case of מי שנולד הספק ברשותו עליו להביא את הראיה
I would like to suggest a possible answer to this question based on the version of the Gemara in BM page 100 that says אלא הא מני סומכוס. That is the gemara there says that סומכוס says ממון המוטל בספק חולקים even when the animal there gave birth in the domain of the seller. Thus even when there is חזקת רשות we find that סומכוס still says his law.
But then it should be a simple case of מי שנולד הספק ברשותו עליו להביא את הראיה
I would like to suggest a possible answer to this question based on the version of the Gemara in BM page 100 that says אלא הא מני סומכוס. That is the gemara there says that סומכוס says ממון המוטל בספק חולקים even when the animal there gave birth in the domain of the seller. Thus even when there is חזקת רשות we find that סומכוס still says his law.
