I suppose you can ask, but I can't see any argument which you could use to get a reduction. You owe them the money. You are able to pay it. You pay it.
If you had a tracker or if you were in financial difficulty, they might do a deal with you, but even then, they are unlikely to.
If, by any chance, it's Bank of Scotland, you could tell them that you are thinking of selling your house. Is there any deal going to encourage you.
Don't start with "I have sold my house. Will you give me a discount?"