In verse 4, the word "reconcile" is ratsah, meaning "to be pleased with, to accept, to favor, to satisfy." The Philistine princes themselves specified what David would have to do to be acceptable to Saul, to regain his favor: He would have to change sides in the middle of the battle. Once he did that, he and his men could slay a large number of the Philistines in a kind of sacrifice to Saul—to show Saul that he, David, was on his side and therefore should be accepted.
So, David would have to make a sacrifice. He would 1) have to turn traitor, 2) have to slay a lot of Philistines, and 3) have to put his own life on the line. In reality, if he would have done this, he would have made himself unacceptable to either side! He would have sacrificed just about everything. This was one reconciliation that David did not want to do! Even though he argued a little bit with Achish to reinforce his cover, saying, "I don't want to return. I want to stay here with you," he and his men went back to the land of the Philistines and did not fight in that battle.
Even in a case like this, some sort of sacrifice would have to be made to bring about reconciliation. Sometimes the sacrifice that is made has consequences itself. We have to make sure that the sacrifice that we make to reconcile with another will not put us into deeper water with God. So when we consider reconciliation, we must think deeply about it; it is not something we should do carelessly or automatically. We must really take the matter to heart to understand fully what we are doing. We have to try to predict what will happen as a result, so that it does not cause greater problems down the road. Reconciliation is not an easy subject and often not easy to do.
Richard T. Ritenbaugh
The Cost of Reconciliation