The most common shorthand for thousands when referring to money is either "grand", "G", or "K". "G" being the abbreviation for grand and "K" being an abbreviation for kilo which is the 1,000 modifier in the metric system.
You could express your statement in several ways:
"I have 100 grand."
"I have 100K" (often spoken as 'I have a hundred k')
"Grand" is more directly related to money references as it wouldn't make sense to say 'There are 60 grand people in the stadium'. Personally, I tend to use "K" over "Grand" in most instances since it is also the most common way to reference thousands from an accounting perspective and also numbers in general ('There are 60k people in the stadium').
As far as I know, "M" would only reference 1,000 if talking in roman numerals. Normally it would be an abbreviation for million.