Partner ended lease early, surprise fee

My partner ended his lease early so that we could move in together. His apartment gave him 2 options: pay 2x rent and be out of the lease completely, or agree to vacate his unit, have it listed, and pay rent until it relet. We chose option 2. The unit was relet within 2 days of being posted so we thought we were in the clear.

Now they’re attempting to charge a $2.5k reletting fee. They never disclosed this fee, it’s not mentioned in his lease. Light research has told us they can charge for the amount to advertise the unit, but this is usually less than $500 and they have to disclose it ahead of time. Also the unit will not be unoccupied – he set his lease to end on the 20th of July, they’ll spend 5 days on repairs, then it’s re-rented.

Any advice on what kind of legal standing we might have to combat this would be appreciated.