Why on earth would you refund him 25% when he chose to get a huge discount in return for committing to a one month stay.
If he is staying for less than a month and you accepted that he could reduce his stay, then you should have recalculated based on your standard weekly discounted rate not the huge 50% discount.
In your situation I would have refused to accept the request to reduce the term of the booking @Huda35 . More importantly I wouldn't have accepted the booking in the first place at such a huge discount - guests who push for huge discounts and want to book off platform are a huge red flag for me.