Yes,subtraction occur in the roman numeral system.It takes place as it takes place in numerical systems.But subtraction occuring in the roman numeral system is difficult then in general numerical system.

Hindu-Arabic numerical system is what we use today i.e. 0 1 2 3 4 5 6 7 8 and 9 Roman numerical system was once used in the past i.e. I V X L C D and M

XVXVX Another answer: 400 in Roman numerals is CD (-100+500 = 400) but 100-1400=400 still because the numeral ups one after subtraction so actually(-100+500 = 400) doesn't exist