An international exchange of valuable art treasures, whereby the Fogg Museum surrenders to the Spanish government the Sahagun Tomb, the most ancient sculptured seculchre of the Middle Ages in Europe, was announced yesterday. The Republic of Spain proclaiming the "sentiments of her gratitude and fraternity which from now on will link the National Archaeological Museum and the Museum of Harvard University," presented the Fogg with a collection of objects of great artistic and archological value. Among the objects received were bronze figurines and columns dating from 1100 A. D.