Brother, the sun existed from the first day. There are three pieces of evidence. First, the word "evening" which means sunset reveals that the sun is the greater light from the first day. It’s obvious that the greater light is present from day one for its purpose to count the days already listed before the fourth day. Second, the use of the “waw-consecutive” on the fourth day continues events that end chronologically after the start of the fourth day. On the fourth day, added is the lessor light of the moon to go with the greater light to complete God's plan for these two lights in the sky. All phrases underlined are a single word from the original language in “waw-consecutive” form. The “waw-consecutive” form tells us that there is a chronological continuation of events with something new added on day four, the moon, which is not necessary before day four, while the sun was necessary to count the earlier days from the first day. Third, there is a different verb used on the fourth day that in the later context with Abraham shows that it means prepared (asah) and not create (bara') as in the previous verses.
And God said, “Let there be lights in the vault of the sky to separate the day from the night, and let them serve as signs to mark sacred times, and days and years, and let them be lights in the vault of the sky to give light on the earth.” And it was so. God made (asah) two great lights—the greater light to govern the day and the lesser light to govern the night. He also made (asah) the stars. God set them in the vault of the sky to give light on the earth, to govern the day and the night, and to separate light from darkness. And God saw that it was good. And there was evening, and there was morning—the fourth day. (Genesis 1:14-19 NIV)
The sun already existed from the first day as explained here to separate the days and added was the moon to the sun to prepare (asah) and not create (bara') the sun that already existed from the first day to decide from now on the seasons of the year in addition to the days from the first day.