The young Netherlands attacker looks set to wave goodbye to Serie A after affirming he only came to Italy to don the Nerazzurri jersey
Touted by some as the new Thierry Henry, Castaignos only made six Serie A appearances last term, with five coming from the bench.
"In Italy I only want to play for Inter, but this is not looking possible now," Castaignos told Sky Sport 24. "The club wants to loan me to an Italian team, but it is a move I would not be interested in.
"We have thus decided that I be sold. I regret not having been able to show my qualities. I am sad, but football is like that and the club and I have made this decision together.
"I regret not being able to go abroad on loan, or play for [former club] Feyenoord next season."
The Nerazzurri missed out on Champions League qualification after a mixed Serie A campaign, in which president Massimo Moratti changed coaches on three occasions.