Tractate Kidushin page 3 side a

The Mishna says a woman can be married in three ways: by money, a document or sex. the gemara kidushin page 3a asks what is the Mishna intending to exclude? exchange. [i.e. kinyan sudar]. you might have thought that we learn marriage by money from a gezera shava acquire from the field of efron and we know that a field can be acquired by exchange. therefore, the Mishna tells us that exchange is excluded. but then the gemara asks but why should it be excluded. answer because exchange works even for things that are less than a pruta a penny and a woman would not agree to be married by something less than a penny. But who cares if she does not like it? After all, no woman can be married against her will even for a million dollars. So, if she agrees to be married (for less than a penny), then she agrees. But that would be so only if the gezera shava is from the Torah. Then it would be absolute. And since exchange would work in the case of a field, then that is that. So now we know why the Rambam held this gezera shava is from the words of the scribes.---------------------The משנה says a woman can be married in three ways: by money, a document or sex. The גמרא קידושין ג' ע''א asks what is the משנה intending to exclude? Exchange, [i.e. קניין סודר]. You might have thought that we learn marriage by money from a גזרה שווה קיחה קיחה משדה עפרון ,and we know that a field can be acquired by exchange. Therefore, the משנה tells us that exchange is excluded. But then, the גמרא asks, "But why should it be excluded? Answer: because exchange works even for things that are less than a פרוטה and a woman would not agree to be married by something less than a פרוטה. But who cares if she does not like it? After all, no woman can be married against her will even for a million dollars. So, if she agrees to be married (for less than a penny), then she agrees. But that would be so only if the גזרה שווה is דאורייתא. Then it would be absolute. And since exchange would work in the case of a field, then that is that. So now we know why the רמב''ם held this גזרה שווה is from דברי סופרים.