I don't remember this kata... at all

or at the very least the indication should be removed that m <= n

Where did you see the indication that m <= n ?
Did you read this?

Note: m, n, f, b, c are positive integers, where 0 <= m <= n or m >= n >= 0. m and n are inclusive.

and the example:

howmuch(10000, 9950) => [["M: 9991", "B: 1427", "C: 1110"]]