I'm not personally aware of dragon-slaying or drunken brawling that resulted in a marriage. However, at least in post-Norman Conquest England (wait, I know something about medieval England? what?), kings and barons could and did arrange marriages between their most loyal retainers and the daughters of other vassals who stood to inherit huge...tracts of land, or other wealth. The purpose was twofold. One, to prevent enemies of the king or baron from gaining either land to supply wealth, or land that could provide a strategic base for attack (in the case of nobles warring with each other). Two, to provide a tantalizing reward for solid service by the vassal--a reward that came without financial cost to the king or baron. Jennifer Ward argues that Henry I would even intervene in inheritance preferences so that a vassal's daughter he wanted to use as a reward would inherit ahead of her brother.

A most spectacular case is William Marshal, the famous English knight, under Henry II and Richard I. Henry promised the faithful but rather impoverished Marshal a marriage to superheiress Isabel of Striguil. This was potentially problematic for Marshal, because Henry was very much at war with his son Richard in Henry's dying days. Would Richard honor the deal? (Would Henry move to disinherit his own son?)

As the legend has it (although this might be propaganda or embellishment; the source is...biased), Marshal happened Richard by surprise one day, and became the only knight ever to unhorse him. Instead of killing the rebellious prince, however, Marshal only slew his horse.

It proved a fortunate--and fortune-ate--decision. Succeeding his father to the throne, Richard honored Henry's promise to Marshal of Isabel's hand in marriage. In one wedding ceremony, Marshal went from not even having enough money to afford the usual festivities surrounding marriage (he had to borrow it!), to basically being the king's wealthiest vassal.

Sources: Jennifer Ward, Women in England in the Middle Ages; David Crouch, William Marshal: Knighthood, War and Chivalry, 1147-1219

From a literary viewpoint, the act of "slaying the dragon" is metaphorical, as is the princess/maiden archetype. I'm thinking partly of Jung but then his is just one view.

The accounts you're probably imagining weren't written to reflect reality in the sense of actual historical events. At least not any more than, say, the book of Genesis, or the myth of Demeter and Persephone.

Mind you I realize I am not answering your question per se, but at a point one must discern the divide between historical accounts and mythological representation.

In European/British Isles tales where the knight or knave is sent on a quest, we're dealing with the same basic stories as say, Perseus, sent by Polydectes (in lust with Perseus' mother) to bring the head of Medusa polydectes or as Jason sent by Pelias for the fleece (guarded by a dragon.)

It's the quest story, Campbell's Hero's Journey if you accept that idea. Which is in turn a metaphor for life--birth, adolescence, obtaining a wound, departure, getting past the gatekeeper, the descent into the abyss, facing the dragon (or whatever), the ascent, the return.

Once upon a time (1066, to be exact) William I of England found himself in possession of rather a lot of land with which to reward his loyal followers, taken from the fallen Anglo-Saxon nobility – especially in the south, where many had died alongside their King on the battlefield.

His successors continued this trend, with William II (aka William Rufus) and Henry I expanding the Crown’s holdings into Wales and Northern England through invasion and pacification. As far as property went, the Norman kings made it rain.

Here we have a clear sense of glorious deed equals demonstrable reward, but where this is relevant to your query is that the marrying off of heiresses was at the discretion of the king.

Women enjoyed greater rights in Anglo-Saxon England and weddings (theoretically) depended on their consent (not that 10th Century England was a Feminist Wonderland or anything, but women were comparatively better off). With the ascension of Henry I, however, they became another ‘trinket’ in the big reward chest.

In 1100, Henry I took control of the whole issue of marriage when it came with financial or political favour. His Charter of Liberties – which he announced upon taking the throne - read:

Any baron or earl who wishes to betroth his daughter or other women kinsfolk in marriage should consult me first, but I will not stand in the way of any prudent marriage. Any widow who wishes to remarry should consult with me, but I shall abide by the wishes of her close relatives, the other barons and earls. I will not allow her to marry one of my enemies.

This was important as it ensured that those nobles loyal to him would beget more nobles loyal to him, by making sure that their daughters and sisters, etc, would be paired up with loyal retainers. The Charter continued:

Any wife of my barons, who becomes a widow shall not be denied her dowry. She should be allowed to remarry according to her wishes, so long as she maintains the integrity of her body, in a lawful manner. Barons overseeing the children of a dead baron shall maintain their land and interest in a lawful manner.

Cynically, the window was now guaranteed her dowry (which included a portion of the late husband’s estate) but in defiance of the Charter she was denied her free will (King John’s abuse of this was one of the grievances that led to Magna Carta) – her remarriage would be at the discretion of the Crown allowing Henry to give away land and riches by other means.

Henry was criticised for doing this across ‘class lines’, marrying men “from the dust, above earls and illustrious castellans,” but he didn’t marry from the bottom to the very top (bad luck for plucky peasant lads hoping to bag themselves a princess). Instead loyal knights were settled into the mid-rung aristocracy, but that this upward mobility were talked about in such melodramatic tones is a testament to how blatant a political manoeuvre it really was – effectively kicking sand in the eyes of the social order.

Professor Judith A Green writes:

Those who received the greatest lands either from the royal demesnes or from the richest heiresses were usually well born… or men to whom he had a special debt.

Similarly, Dr Jennifer Ward writes:

Henry I made extensive use of the marriage of heiresses and widows in rewarding his curiales [courtiers], sometimes even passing over male heirs.

So no dragons need be slain, but Norman and Angevin monarchs used marriage as way to directly reward their loyal followers, boosting their status, lands and wealth. Much like fairy tales, the bride's feelings played no part.

And they all lived happily ever after (provided they were the king’s cronies).

Sources: The Aristocracy of Norman England by Judith A. Green, The Government of England Under Henry I by Judith A. Green., and Women in England in the Middle Ages by Jennifer Ward.

It was known after wars for fathers to negotiate their daughters marriage as part of a treaty or a political move. Treaty of Troyes included a marriage and promise heirs will inherit the throne.

Diarmait Mac Murchada who was exiled from Ireland promised his daughter Aoife MacMurrough and some lands to Norman mercenaries (she married Richard de Clare through this arrangement) to help him invade Ireland, but I've read she still had legal right to choose who to marry (but hard to know how true that freedom was). There are romantic style painting of their marriage and there could be connection as Norman first romanced lot of knights and Arthurian tales. But in this case and others it might be down to what laws there were under arranged marriages too during medieval times and the different regions.

I'll observe that in Norwegian fairy-tales, when a king is involved, the action does not take place "at court" or "in the palace" but "at the king's farm", "kongsgården". Which suggests that for "king" you should read "wealthiest local landowner" or perhaps "magnate". Although I don't know of any examples, that does make it a bit more plausible for a poor man to marry the heiress for services rendered - no European diplomacy is going to be involved.

Similarly, in German fairytales, a woman who marries "the prince" might not need a very tall tower to see everything her husband's father rules.