The boy was born earlier this year to a 22-year-old woman and 24-year-old man in Ghent in Belgium's Flemish region, and adopted by the Dutch couple four months ago. Dutch media have said baby's biological parents were paid between 5,000 and 10,000 euros.
The Ghent couple has launched legal proceedings have the baby returned, their lawyer said.
A lawyer for the couple told Flemish VTM television earlier that the couple felt bad about their decision and had come under pressure from family and friends to change their mind. He also said their financial situation had improved.
Earlier reports said the parents faced five to 10 years in prison as the adoption went ahead without official authorization.
The Dutch couple reportedly want to legally adopt the boy.
A court in the Netherlands has ordered an inquiry and has ruled that the baby must be temporarily placed with another adoptive family.
