Sales Agreements: Dealer Wants More Money

My question involves a consumer law issue in the province of ontario,

so went through the usual agreements with the dealership. they basically told me id get a good offer if i was to take the car then and there. rushed paperwork and so on. once all the hidden fees were put in final number looked high. I said it was too much. they got the number down to where i wanted it.
problem is they miscalculated the bill to get this final number. months later they called and want the difference and now they are threatening to bring me to court.
is their anything i can do to protect myself here or am i responsible because i didnt do the math for them?

any advise would great,

thanks

Sales Agreements: Dealer Wants More Money