Not a teacher
You are right: The word increment/decrement is usually used in computer science and means to increase/decrease a certain amount by one. However, if you go to a store to buy something expensive you might ask the dealer "Can I pay this in increments?". That means you are asking for an option to pay in parts; for example you could pay him 1/12 of the product's cost every month for one year.
Since the person in your example doesn't need to study for one more full year he is asking if the University offers a reduction of the fee for a single semester.
Student or Learner