While King Fernando was in Salvaterra [de Magos], one of his townships near the River Tagus, he started to feel unwell and was quite poorly. When he heard the news that the Archbishop of Santiago was travelling to see him as ambassador acting on behalf of the King of Castile, with regard to the matter of his daughter's marriage to the King of Castile, he arranged for him to be met at the border by the Bishop of Lisbon, Don Martín. They both arrived there in March 1383. After the warm welcome which the king gave to the bishop and his ambassador, they spoke for several days about all the matters connected with the arrangement, discussing the marriage as well as the accession to the throne, should King Fernando happen to die without a son.

One day everyone was informed, in the presence of the king, that the conditions of the marriage were as follows: the archbishop should take charge of the aforesaid princess in the name of the king his liege lord, when the time came for her to be taken to her husband; the King of Castile should arrive between Elvas and Badajoz to receive her as his wife; before she was handed over to him, there should be displayed the dispensation of the Pope that freed them from the obstacle caused by the family ties between them. In addition, as the princess was not yet twelve years old, it should be announced by whoever had the authority that she was entitled to marry; furthermore, the King of Castile was to take her to Badajoz, where their wedding and the celebrations connected therewith would take place with all due honour, and the king would welcome her again with a formal promise of marriage.

Besides that, King Fernando was to give the King of Castile twice the amount of money that had been given as dowry to King Alfonso [XI], the grandfather of the present King Juan, when he married Dona Maria, the aunt of King Fernando, and this was to be paid within three years. Also, the King of Castile was to give his wife all the towns and villages which belonged to his mother, Queen Juana, at the time of her death.