@Cherry99 I think what he's doing is dodging the expiration date of the coupon. If you book with a superhost coupon and cancel, the money goes poof. However, if you change the dates the value should stay intact. It could be the same for a coupon for a canceled reservation or some kind of reimbursement from AirBnB. I bet that's it.
This time you should just say no. At a minimum, ask him why he's doing this, and explain what he's doing to your calendar. There may be another explanation(?)
Good luck.