I am far from an expert on duels, and am rather an amateur (at best) historian of medieval Scandinavia and Greece and the relationship between the two, or more accurately, am a fan of books about vikings, the Byzantine Empire, and vikings in the Byzantine Empire (aka the Varangian Guard).

However, at your description of a "professional duelist", I am reminded of a character from Egil's saga by the name of Ljot the Pale, a berserker who abused the concept of holmgang (the medieval Scandinavian version of a duel) against opponents he suspected of being both weak and rich to become quite wealthy himself.

From Chapter 67 of Egil's saga:

Then Gyda went to Egil and said: 'I will tell you, Egil, how things stand here with us. There is a man named Ljot the Pale. He is a Berserk and a duellist; he is hated. He came here and asked my daughter to wife; but we answered at once, refusing the match. Whereupon he challenged my son Fridgeir to wager of battle; and he has to go to-morrow to this combat on the island called Vors. Now I wished, Egil, that you should go to the combat with Fridgeir. It would soon be shown if Arinbjorn were here in the land, that we should not endure the overbearing of such a fellow as is Ljot.'

Knowing that Ljot will absolutely wreck Fridgeir in combat, they ask Egil to stand in for him, who is a big murderous bastard himself, and he obliges:

They now start, and soon come to the island. There was a fair plain near the sea, which was to be the place of combat. The ground was marked out by stones lying round in a ring. Soon came thither Ljot and his party. Then he made him ready for the combat. He had shield and sword. Ljot was a man of vast size and strong. And as he came forward on the field to the ground of combat, a fit of Berserk fury seized him; he began to bellow hideously, and bit his shield. Fridgeir was not a tall man; he was slenderly built, comely in face, not strong. He had not been used to combats. But when Egil saw Ljot, then he sang a stave:

'It fits not young Fridgeir

To fight with this warrior,

Grim gnawer of shield-rim,

By his gods who doth curse.

I better may meet him,

May rescue the maiden;

Full fearsome he stareth,

Yet "fey" are his eyes.'

Ljot agreed (whether he wanted to or not, you cant just go around challenging anyone you want then just refuse a challenge of your own) and the battle was on:

Then Ljot came forward on the field and declared the law of combat, that he should ever after bear the name of dastard who should draw back outside the boundary stones that were set up in a ring round the field of combat. This done, they closed, and Egil dealt a blow at Ljot, which Ljot parried with his shield, but Egil then dealt blow upon blow so fast that Ljot got no chance for a blow in return. He drew back to get room for a stroke, but Egil pressed as quickly after him, dealing blows with all his vigour. Ljot went out beyond the boundary stones far into the field. So ended the first bout. Then Ljot begged for a rest. Egil let it be so. They stopped therefore and rested. And Egil sang:'

Free-handed gold-giver,

Back goeth yon champion,

In craven fear crouches

This wealth-craving wight.

Not strongly fights spearmen

His strokes who delayeth.

Lo beat by a bald-head

This bragging pest flies.'

Egil, being the star of this story, beats that ass:

These were the laws of wager of battle in those times, that when one man challenged another on any claim, and the challenger gained the victory, then he should have as prize of victory that which he had claimed in his challenge. But if he were vanquished, then should he ransom himself for such price as should be fixed. But if he were slain on the field, then had he forfeited all his possessions, and he who slew him in the combat should take his inheritance. This was also law, that if a foreigner died who had no heir in the land, then that inheritance fell to the king's treasury.

And now Egil bade Ljot be ready.

'I will,' he said, 'that we now try to the uttermost this combat.'

Ljot sprang swiftly to his feet. Egil bounded at him and dealt at once a blow at him. He pressed him so close, that he was driven back, and the shield shifted from before him. Then smote Egil at Ljot, and the blow came on him above the knee, taking off his leg. Ljot then fell and soon expired. Then Egil went to where Fridgeir and his party stood. He was heartily thanked for this work. Then sang Egil:

'Fall'n lies the wolf-feeder,

Foul worker of mischief:

Ljot's leg by skald sever'd

Leaves Fridgeir in peace.

From the free gold-giver

Guerdon none I seek me,

Sport I deem the spear-din,

Sport with such pale foe.'

The historicity of the sagas has been debated by far smarter people than I, and I have nothing to add to that, other then that given the attention to these specific rules and conditions of the duel, I would believe that even if Ljot the Pale didn't exist, that his scam did:

Ljot's death was little mourned, for he had been a turbulent bully. He was a Swede by birth, and had no kin there in the land. He had come thither and amassed him wealth by duels. He had slain many worthy landowners, whom he had first challenged to wager of battle for their lands and heritages; he had now become very wealthy both in lands and chattels.

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EDIT: weird phone format issues