I am not sure how to say this simply but basically I think there is some proof that the Rambam does not hold like the Rashbam but that אין שמין לגנב means he must pay כלים שלמים or כסף.
Reb Chaim says there is  a doubt about this and Rav Shach brings a proof that the Ramabm does hold with the Rashbam that the גנב can pay שווה כסף
The proof I have that the Rambam does not hold by the Rashbam is from הלכות גניבה פרק א' הלכה י''ד.
My reason is simple. If the Rambam would hold by the Rashbam then why does he not write simply if the thief broke the vessel we evaluate it according to the time of העמדה בדין? Why does he divide halacha 14 into two parts? One is which the value of the vessel went down and we go by שעת הגניבה and part two is where it went up in value and then he broke it and we go by שעת העמדה בדין?
Part one  by itself is not a question on the idea that that Rambam hold from the Rashbam because it only is referring to a case where the vessel was not broken. But if we look at halacha 14 in its entirely it is obvious something is missing in part one--then case where the vessel went down in value and then it was broken. If the Rambam really would be holding from the Rashbam then he would say if the vessel went down or up in value and then it was broken we go by שעת העמדה בדין.
So instead according to the way I see it, אין שמין  has nothing to do with the time of evaluation but the fact that the thief must give back כלים שלמים or כסף.
However Rav Shach's proof is also valid. Thus we are left with a doubt about what the Rabam really holds.