Meanwhile, King Fernando had received the agreement of those in his Royal Council that in order to wage war against King Enrique he could not find a better way than to suggest to King Pere of Aragon that his daughter, Princess Leonor, who had been betrothed to Prince Juan, the son of King Enrique, should marry him, King Fernando. By such a marriage, he planned greatly to advance his cause, along with the other forms of assistance which he had. That was because, with the Emir of Granada on the one hand, with the King of Aragon on the other and, on his own account, with the forces and places which had gone over to his side, he concluded that the situation was very propitious for what he had begun to be completed all the earlier.

Accordingly, he requested the hand of Princess Leonor in marriage, sending as envoys Baldassare di Spinola, Alfonso Fernández de Burgos and Martín García, who were knights in his Royal Council. After they had spoken to the King of Aragon, the latter was pleased to arrange her marriage to King Fernando, sending one of his knights, Don Juan de Villaragut by name, with full powers to confirm the marriage. He arrived in Lisbon, where King Fernando was staying, and after drawing up the agreement, the king was married by proxy in the Church of São Martinho in Lisbon, because at the time the king was lodging in the Palace of the Princes, which is close to that church.

The treaty contained one condition, namely that, with all his might, the King of Aragon would help him to wage war against the King of Castile for a full two years and that 1,500 lances would be paid at King Fernando's expense. As it was necessary for these men-at-arms to receive their pay in a coinage that was customarily used in the kingdom of Aragon, this treaty laid down that King Fernando would send to Aragon sufficient gold and silver bullion to enable the minting of enough florins and reales to pay the forces who would have to wage war but who would not be allowed to forage for food off the King of Aragon's land as they marched across it once the war had begun.