first of all, if a loan allows early payoff without a penalty, then it's your prerogative to take that option anytime you feel like it with absolutely no moral qualm. that's the point of written down contracts. you are under no obligation to discuss your future financial plans with the sales staff, nor what strategy you plan to take. they in turn are free to make any deal they wish, and if they offer a deal whereby you have an option, they have offered you a deal whereby you have an option. you don't need to lie, but it's really irrelevant because the only thing that matters is the contract - if either side actually cared about something that isn't written down, then shame on them. "assumptions" is no part of it.
secondly, it factually just doesn't work that way. you could straight up tell them you want to pay it off tomorrow. they won't give a shit. if they "make money on the financing" via some incentive, they get paid when they close the loan regardless of when you pay it off. if you pay it off early they don't even know about it and could care less. and if the finance company cared, it wouldn't be an option or it would have a penalty.
you don't need to, nor should you, "stoop to their level". simply go in, pick a car you like, tell them what you want to pay. if they say "there's a better price if you finance" then either take that deal, or counter-offer the same in cash, or don't and leave. if they say, sorry i can only do that if you finance, then either take the financed deal (sometimes the rates really are quite good) or ask if it has a prepayment penalty: "so, if i pay that off in full next month, there's no penalty and you still give me this price?". then make them show you in the paperwork where it says that.
it's fine to not want to play games with deceptive dealers, but you are certainly under no moral or ethical obligation other than to hold up your end of whatever deal that is made.