When, whether talking about money or not, have you ever seen someone write 100M to mean "a hundred thousand"? That would be completely ridiculous. Roman numerals are not used for that.
That guy's response makes no sense to me. I worked in finance and banking for five years and didn't even once see M used to represent thousand. Perhaps times have changed and that used to be common. k (thousand) and M, m or MM (millions) were used though.
