Most Genos prices are around 3300 pounds - some a bit higher, some lower- but depends on the detail of what is included. This means they are giving you about 1500 pounds for your in part exchange Tyros. That seems very fair, since they probably will sell it for no more than 1800 - 2000.
You could probably try selling your T 5 personally, and maybe squeeze a little more out of it, then go without a part exchange and try to squeeze the dealer down a bit, but that is an awful lot of hassle for very little gain, and no certainty it will work out.
Mike