The 19-year-old is reportedly close to the San Siro exit door, with the Dutch club prime favourites to seal his signature, despite interest from the Premier League
The 19-year-old made the move to San Siro from Feyenoord last season. While impressing early on, he failed to secure playing time; making only six Serie A appearances as the Nerazzurri finished a lowly sixth.
Both Twente and Premier League outfit QPR are reportedly interested in the Netherlands youth international, who is linked with a move away from Milan.
The report states Castaignos has already reached an agreement with the Dutch club to return to the Eredvisie, with a four-year contract and €5 million deal mooted.
QPR have also submitted a bid, thought to be around €4 million, which was rejected earlier this week by the Serie A giants, despite reported interest from a host of Premier League clubs including West Ham and Everton.
However, Munsterman believes that a potential transfer will not be straightforward, as he seeks to iron out the final details of the contract.
"We will meet with Inter at the [negotiating] table. It will not be so easy." the club supremo was quoted as saying by De Telegraaf.
Munsterman had earlier revealed Castaignos' wish to play in Enschede next season.