Don't give in Kaz, you don't deserve to be shafted like that. If the new buyer really wants the business, they will pay the full amount.
PS, 100.000 pounds could buy my condo here in Vancouver outright. That's $186.00 US dollars or $210.858 Canadian. Certainly a LOT of money that you deserve. You put your blood sweat and tears into that business and you are passing it on with all your contacts and staff that made the business the success it is. For anyone to ask for a rebate after making the initial offer, well I know where I'd tell them to go.
I say, stick to your guns and don't back down from the original deal.
Cheers,
Sean