I think that Dr. Smedes meant this: When we forgive, often times the injustice comes from someone with whom we have been in a relationship (a family member, a business colleague, for example). The fact that we have to forgive means that there was an injustice that could strain a relationship. As we forgive, we become open to receiving that other person back into our lives. This does not mean that we will receive an apology and a new, trusting relationship (because our forgiving does not automatically mean that the other will now be fair). Yet, forgiving does make us open to this possibility of the other accepting our forgiving and thus becoming more fair. I think the key to understanding Professor Smedes’ sentence is his word “hope.” As we forgive, we are open to the other’s changing. We wait in hope for that change.
For additional information, see Why would fairness be given to someone who has been constantly unfair to me over the years?