It is a thoroughly well-known feature of the halakha that one may not use money on Shabbat, and there are maybe a dozen questions on this website alone that seem to presuppose familiarity with that fact. But why? The use of money is not listed in Mishna Shabbat 7:2, nor does the act of commerce appear to constitute a subset of any of the prohibitions that are mentioned. Is it prohibited under the principle of shevut? If so, I note that it is nowhere mentioned in Mishna Beitzah 5:2 either.
I would assume that the act of purchasing something is what is forbidden (or perhaps the act of earning money?), which makes handling money forbidden under the principle of כלי שמלאכתו לאיסור, although I would very much like to see that written down somewhere.
What is the earliest source for the prohibition of spending/earning money, under what principle is the handling of money forbidden (shevut or muqtzeh), and were there any dissenting voices on this matter?
Note: The prohibition of trade already seems to be inferred by Nehemiah 13:15-21, although note that much of his objection concerns the use of animals, the travel to Jerusalem and the entering of a walled city. His prohibiting Tyrians from doing likewise would seem to presuppose a prohibition of buying as well as selling, since Phoenicians are not bound by the laws of Shabbat. If his enactment has a textual basis in the Torah, I do not know what it is.