I think one interpretation of the difference in the scenarios between 21-24 and 25-26 is the difference between subjective and objective wrong. Many of our arguments and disagreements with people are subjective. By this I mean that an outsider might have trouble discerning who is at fault, either because there is fault on both sides or because the two sides disagree on exactly what happened. These arguments drag on and on because each one thinks the other should apologize first. In 21-24, Jesus is saying, "Get over it. Be the bigger person and go apologize."
In verses 25-26 there is a more objective wrong. One person borrowed a certain amount of money which is not in dispute. The borrower is objectively wrong for not repaying the money on time. Therefore the borrower should work things out before they come to a head or pay the consequences.
The conclusion from these verses is, I think, that no matter whether you are in the wrong or not take the first step in working things out. It is more important to be at peace than to be right. If you are not at peace with others, you are not at peace with God.