Hadouir agreed a two-year deal with Alemannia Aachen over the weekend and has turned his back on Boro and staying in Holland with Feyenoord.
The 28-year-old was on Teesside last month when he was shown around Middlesbrough's facilities by manager Tony Mowbray.
Despite a willingness to move to the North-East, however, Hadouir was forced to wait while Mowbray went about trimming the club's wage bill.
In the mean time, though, he has grown impatient and is thought to have decided on a new career in the Bundesliga with Aachen.
Hadouir became a free agent after his contract with Roda JC expired and had been keen to test himself in England, where he had been impressed by Mowbray's vision for the club.