Al-Hadari regretted leaving Ahli without approval as he returned to Egypt, claiming that he terminated his contract with Sion.
However, Constantin revealed that the 35-year-old keeper has only returned for clearing up personal matters and should re-join Sion.
"He has been under a lot of pressure and received terrible threats, his family was insulted, some considered him a traitor, so he had to go back," the Sion president was quoted by Le Matin newspaper.
"According to the contract he signed, the player belongs to us, which means that he cannot play for any other club.
"If Ahli chose to play him, they shall be deemed as losers automatically."
Ahli will decide on Tuesday on the action to be taken towards Al-Hadari, who said that he is ready to accept the consequences of his controversial move.