It was customary in Christian Spain that a fifth of the spoils would go to the King. The same proportion is required in the Koran to be given to the community as well.
(Sura 8, Line 42: “...and know that whenever ye seize anything as a spoil, to God belongs a fifth thereof, and to His Apostle, and to kindred and orphans...”)
And he came up with them between Atienza and San Estevan de Gormaz, as they were carrying away a great booty in captives and in flocks, and there he had a brave battle with them in the field; and in fine Rodrigo conquered, smiting and slaying, and the pursuit lasted for seven leagues, and he recovered all the spoil, which was so great that two hundred horses were the fifth, for the whole spoil was worth a hundred times a thousand maravedis.
And Ruydiez took the tribute and offered the fifth thereof to the King, in token of his sovereignty; and the King thanked him, but would not receive it, and from that time he ordered that Ruydiez should be called the Cid, because the Moors had so called him.
And he gave order that all the spoils should be heaped together, both what Alvar Fanez had brought, and what had been taken in the castle; and he said to him, Brother, of all this which God hath given us, take you the fifth, for you well deserve it; but Minaya would not, saying, You have need of it for our support.
And the Cid divided the spoil among the knights and foot-soldiers, to each his due portion; to every horseman a hundred marks of silver, and half as much to the foot-soldiers: and because he could find none to whom to sell his fifth, he spake to the Moors of Castrejon, and sent to those of Fita and Guadalajara, telling them that they might come safely to purchase the spoil, and the prisoners also whom he had taken, both men prisoners and women, for he would have none with him.
Then the Cid called unto Alvar Fanez and said, Cousin, you are my right hand, and I hold it good that you should take of my fifth as much as you will, for all would be well bestowed upon you; but Minaya thanked him, and said, that he would take nothing more than his share.
And of his fifth of the horses which were taken that day, my Cid hath sent you these, as to his natural Lord, whose favour he desireth.
Great booty came to him, and moreover the Cid sent him the tithe of his fifth.
And he went out against them, and smote them, and hath sent you these two hundred horses from his fifth.
When the Infantes of Carrion, Diego Gonzalez and Ferrando Gonzalez, saw the noble present which the Cid had sent unto the King, and heard how his riches and power daily increased, and thought what his wealth must needs be when he had given those horses out of the fifth of one battle, and moreover that he was Lord of Valencia: they spake one with the other, and agreed, that if the Cid would give them his daughters to wife, they should be well married, and become rich and honourable.
So great was the spoil that six hundred horses fell to the Cid as his fifth, beside sumpter beasts and camels, and twelve hundred prisoners; and of the other things which were taken no man can give account, nor of the treasure which the Cid won that day in the Campo del Quarto.