Raphael ranks among the greatest artists of all time. Yet they say he once sold what has since become a masterpiece for a bit of food. One evening the great painter saw a mother singing to her small child. He was inspired to paint the scene ─ but there was no canvas to paint on. Raphael discovered an old barrel nearby and used its round cover as his canvas. The story is that he sold the painting to a bar keeper for a meal worth less than fifty cents, though he was not a poor man. The Italian government sent the painting to the New York World’s Fair in 1939, where it was exhibited. It now hangs in a palace in Florence, Italy and is valued at over $1,000,000!
1)they say he once sold what has since become a masterpiece for a bit of food.
This sentence seems strange when it comes to "tense agreement". The main clause was put in the past tense, but the subordinate one was in written in the present perfect, which means "present". I don't think the tense of main clause could precede that of dependent clause.
By any chance is it right? If so, how do I understand it?