As explained by the previous answers, you have two basic options and many different ways to implement each.

The first option is to use the Arabic words. This is not unusual in this kind of literature. For example, there is a concept in Anglo-saxon law similar to the concept of دية called "wergeld." If you break the word down, it literally means "man gold" (or money for the life a human being). This concept is usually referred to in English by this Anglo-saxon term, not in a Modern English substitue. Likewise, you can use the Arabic word DIYA as is.

On the other hand, it is not incorrect to use a Modern English word. The concept translates fairly well because it is not peculiar to Arab culture or Islamic law. It is, however, framed differently in different legal systems and different cultures. Even in one country, like the US, you can find different terms in different states. In most states in the US, this concept is practiced under the heading of "manslaughter damages."

My key point here is to choose a method and stick to it throughtout your piece to achieve proper consistency. You can implement either one of these two methods in different ways and possibly combine the two approaches by using parenthetical explanations.

without fuller context, I could tell what is meant by فصل. Until you post fuller context, I would guess that فصل here means arbitration.