Dear Reb Aharon Beracha

Why is the order of TOSFOS BABA KAMA 3b DH LE-FOTRO and DH AMAI the wrong way round? (Of course because it's PURIM and "VE-NAHFOCH HOO!!!!!")

Another possible answer is given by CHESHEK SHLOMOH (printed at end of Gemara p. 80). We need to understand first TOSFOS DH LE-FOTRO in order to understand correctly the Gemara's question "Why is it called a Toldah of Regel?" Without Tosfos, one would have thought that Tseroros is not similar to Regel and therefore the Gemara asked why it is called a Toldah of Regel because it is similar to Keren since Tseroros also pays Chatzi Nezek. Therefore, the first Tosfos explains that in fact Tseroros is similar to Regel because it should really have paid Nezek Shalem like Regel, and only because of the Hilchesa does it pay Chatzi Nezek.

Now that we know that Tseroros is similar to Regel one can no longer understand the Gemara's question "Why is it called a Toldah of Regel ?" because in fact one would say that it is called a Toldah because of it's similarity to Regel. Therefore TOSFOS DH AMAI explained that even though it is similar to Regel nevertheless the intention of the Gemara's question was that since the Din is different from Regel(i.e. Regel pays Nezek Shalem while Tseroros pays Chatzi Nezek) one therefore should call Tseroros by some different special name(but the Gemara never intended to ask why Tseroros is not called Keren because we knew all along that it is not similar to Keren)

In summary, TOSFOS DH LE-FOTRO had to come first in order to explain that Tseroros is similar to Regel. Only once we knew this can we understand properly TOSFOS DH AMAI that even though they are similar, nevertheless the Gemara still asks why it is called a Toldah of Regel because it should have been called by a different name since it's Din is not the same as Regel. (see also YAD DAVID who answers similarly to CHESHEK SHLOMO but explains more. Your question is also mentioned in PNEI YEHOSHUA)

R. Dovid Bloom