Thing is, lots of people on this forum are against thinking for yourself and are more centered towards taking every analogy and ruling from islamic texts and scholarly quotes/fatwas.
Quoting scholar's fatwa is the safest way of protection from accountability on judgement day. But if you want my opinion then...
First, any issue which isn't prohibited by definitive evidence isn't haram, however, it may be closer to being haram if (operative causes) illat of undefined matter (muqees) is similar to the defined matter (muqees alaihi) then injunction concerning muqees alaihi will be transmitted to muqees as well.
Second, the person who is selling this food knows its extra cost because of extra fries/ingredients used in this food by mistake and he doesn't ask for extra money, so you can eat it
or he can take it back and should serve what you have asked for.
But if the seller of the food doesn't know about this mistake and you know, then you should return it and ask the same food for what you've paid/to pay.
Or neither he nor you know about this mistake and you realise it after eating food then if you have money are bound to pay full cost as it's against taqwa.
And Allah swt knows best