Nissan X-Trail: Error in price negotiation with dealer

We recently negotiated the price on a demo Nissan X-Trail Ti with a trade in and finance deal of 1 per cent, and contracts were signed on Monday 7th December 2015. Then on 12th December the new car sales manager called to advise that the finance side of things was ok, but they require an extra $1880 due to an under pricing of the vehicle in conjunction with the 1per cent finance deal that should not have been offered, although the sales person was negotiating our trade etc. of the said vehicle with his sales manager. In a nutshell they want me to pay an extra $1880 because they made an error in the current deals offered and now want to renege on the negotiated deal. What leg do we stand on? Please advise.