Learn Spanish (Spain): Si negocias el precio con el dueño, puedes comprar el coche por menos dinero. - If you negotiate the price with the owner, you can buy the car for less money.

In Spanish, por is used to express an exchange or a price. Because you are trading the money to get the car, you must use por menos dinero. Using para here would be incorrect because para is for destinations or final recipients, not for amounts paid in an exchange.

I thought the word si often triggered the subjunctive. Why is negocias in the normal present tense?

When you talk about a real or very possible condition, si is always followed by the indicative (normal) present tense. You only use the subjunctive after si when talking about unlikely or imaginary situations, like saying if I were a millionaire.

Could I use propietario instead of dueño in this sentence?

Yes, you could say con el propietario. Both words mean owner and are completely interchangeable here. Dueño is just slightly more common in casual everyday conversation, while propietario can sound a bit more official or formal.

Does the word menos ever change its ending to match the noun it describes?

No, menos is invariable, meaning it never changes form. Whether it is describing a masculine singular noun like dinero or a feminine plural noun like monedas, it always stays exactly the same.