Rashi (DH Menalan) explains that we cannot learn Gilgul SHevu'ah on a Ta'anas Safek of Mamon from Sotah with a Kal v'Chomer, since there is a Pircha: Perhaps only Sotah, where the main Shevu'ah is a Safek, can drag along a secondary Shevu'ah for a Safek. By Mamon (which we learn from Sotah), the main Ta'anah is Vadai and so the secondary one must also be Vadai.
If so, how does the Gemara then go and learn Gilgul Shevu'ah for a Safek of Mamon from a Mah Matzinu ("Ne'emrah Shevu'ah bi'Fnim etc.," -- see Tosfos who calls it a Mah Matzinu). Why can't the same Pircha be asked on the Mah Matzinu?
Y. Taub,
Yerushalayim
Tosfos (DH "Nemrah") explains that the Kal v'Chomer was strong enough to teach us that the Gilgul Shevuah should apply by Mamon as well as Isur. The entire point of the Mah Matzinu is that the Torah used the word Shevuah by both topics in order that we should discount the apparent question that a Safek is not like a Vadai. [In other words, the question is only a question on the general Kal v'Chomer, with the Mah Matzinu being the specific answer to that question.]
Kol Tuv,
Yaakov Montrose