Often people dream of fame, wealth, eternal youth and often in legends there are characters (the Devil, fairies, spirits) who are ready to offer men such goods, but for a very high price - usually it is the soul of the person.
In the legend of the Devil's bridge the Devil promises to tell a craftsman how to make a very strong bridge, so the water wouldn't sweep it away if the manages to make the Devil's image both visible and invisible. If the craftsman fails to do that the Devil would take his soul. On the one hand it is normal to expect that whenever you make a deal with such creature, it will ask a price. In legends with the same story line however the price is always very high. And the main characters usually try to outsmart the creature (the Devil) so that they receive the good, the advise, the wealth, but without giving him their souls. In the Bulgarian legend of the Devil's bridge the craftsman managed to fulfill the Devil's condition so he saved his soul, but in the Polish legend of Mr. Twardowski - the main character was closer to loose his soul because he couldn't be smarter than the Devil. The adventures of Mr. Twardowski are very interesting to see - and the legend shows how tricky the Devil could be when he wants to have what is promised to him.
Another legend that follows such plot is the legend of Skarbnik - in one version of it the spirit Skarbnik, who is guardian of minors offers wealth to one miner if he gives to Skrabnik "something he didn't have when he left the house" - this appeared to be the miner's son and if you want to see if the miner managed to save his son from Skrabnik you need to check the legend.
Another aspect of such stories is when people ask from mythological creatures things out of being too spoiled. Such a story is behind the legend of Mount Sorapiss and Lake Misurina, in which a young girl sacrified her father to be turned into a mountain by a fairy, just because she wanted the fairy's mirror.