Is it permissible for a fasting Muslim to delay Maghrib prayer until after finishing Iftar, knowing that he will pray it alone?

If it is possible to combine congregational prayer with breaking the fast, then that is the preferred option. However, if it is not possible and the fasting person's soul yearns for food, then he may eat, even if he misses the congregational prayer. This is based on the hadith (definition): "When the food is served and the prayer is due, begin with the food," and the hadith: "There is no prayer in the presence of food." The meaning is that if food is served and one's soul yearns for it, it is permissible to miss the congregational prayer. This applies to missing the congregational prayer, not missing the prayer's time. If the time for prayer becomes so narrow that there is only enough time left to perform it, then it is not permissible to engage in anything other than prayer.