The 140 mph speedometer was introduced part-way through the 1989 model year after the existing stock of 85 mph speedos were used up. Check your manufacture date on the doorjamb sticker, it's probably late '88 or early '89 which would explain the 85 MPH speedo in your Mustang.
If the buyer doesn't believe you, this info is available on other Mustang web sites he can check out for himself, such as
'Mustang GT' (
www.mustanggt.org ).
Good luck with the sale.