I think it is a lot simpler than that.
Dortmund get £65m in the summer or a hell of a lot more in January. If you were selling, which option would you prefer?
Depends, on how much more, and whats the knock-on worth to them.
65m is still a 50m profit on what they paid for him. An extra 30m they light get in January, assiming anyone is weiilling to pay it rather than waiting, may well ve worth less to them with no time to buy a replacement and missing out on CL as a result.