Maatsen joined BVB on-loan in January to the end of the season.
Sky Deutschland's Patrick Berger is reporting: "Ian Maatsen wants to leave Chelsea on a permanent deal in summer.
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"According to our information Chelsea is already informed about his plan.
"The left-back (2 goals, 2 ass. for BVB) feels very comfortable in Dortmund and is keen to stay. Release clause: 35-40m."