Look at the lease!
If the lease requires this on a transfer then you are stuck. You may be able to contest the size of the fee at the LVT (others will tell you if you can) but there would be a timescale involved and that could put off your buyer. Moral generally in this sort of case is "pay up and say nothing" because if I was acting for a buyer and I found out that you were arguing about the fee (or I found out how much the fee was) I would be telling the buyer seriously to consider not proceeding because there was a rip off landlord involved.
As a conveyancing solicitor I believe the information given in the post to be useful (provided it relates to property in England & Wales) but I accept no liability except to fee-paying clients.