Once there was a poor fisherman with three sons. One night, he was trying unsuccessfully to fish, and a mermaid approached him. She told him that if he and his wife would give them their unborn son at the age of 21, she would make them rich. Neither he nor his wife has any problem agreeing to this, and the next night he finds more gold than he can carry, and becomes very prosperous.
He went on to have a total of seven sons, but was burdened by the knowledge that he must lose one of them. Finally he revealed to the son in question that he had sold him to a mermaid and the time was nearly up, so the son decided to leave. Although his father was troubled at his going, the son took expenses and food and left.
The son traveled until he was tired and sat down to rest, and saw a lion coming towards him. He thought he was dead for sure, but the lion offered to carry him. The lion carried him a long way and brought him to the house of a shoemaker.
The shoemaker told the traveler that the next day, the King's daughter was going to be devoured by a sea monster, unless someone could defeat it. With nothing but an old rusty sword used for cutting vegetables, the son took a boat to where the Princess was waiting. No one but the young man had the courage to fight the monster, and he killed it. The Princess would only marry him.
Shortly after, the mermaid approached, and revealed that her intention all along had not been to kill him, but to bring him to his present Kingdom to rescue the Princess. The mermaid left and was not seen again.
Siren by Jean-Baptiste Monge
"Andromeda's Fate" by PaulHectorT
Source: Surlalune's Mermaid and Other Water Spirit Tales From Around the World, summarized by me