"The Fair Flower of Northumberland" (Roud 25, Child 9) is a folk ballad.
The knight persuades the Earl's daughter, the titular fair flower, to free him, promising to marry her in Scotland.
[2] Parts of it parallel "The Nut-Brown Maid", where the hero tells the heroine that he has nothing to give her, and is plighted to another women, but in that ballad, that is only a test, and he reveals himself as her true and wealthy lover.
[4] As an example of the same elements in other traditions, several Serbian and Bulgarian ballads about the epic hero Prince Marko have a fairly similar plot: Marko is imprisoned in an Arab country, but is secretly aided and set free by the daughter of the Arab king in exchange for his promise to take her with him and marry her - a promise that he soon regrets and breaks.
He then builds numerous monasteries, churches, fountains, roads and other public facilities in her memory, striving to atone for his sin towards her and God.