I guessed that from context and was just making fun of it. Writing it that way is definitely confusing and shouldn’t be done. “mm” is internationally recognized as “millimeters”. In financial papers, sometimes, Roman numerals “MM” are used to indicate millions, but then it should strictly be capitalized. In my opinion, for general use, number followed by a single “m” conveys the idea best ($40m), and followed by “k” for thousand ($40k). Use capitals for more formal settings ($40M and $40K).