"He transfers one hundred euros."
Translation:Er überweist hundert Euro.
because usually numbers aren't capitalized, but "Million", "Milliarde" etc. are treated as nouns, and therefore capitalized.
You can guess this a little when looking at English. You can say "a/the million", but you usually don't say "a twenty" (of course you can, if you talk a bout the number twenty written somewhere, but in this case you would capitalize "die Zwanzig" in German as well).