"Are you paying for the dessert?"
Translation:¿Tú pagas el postre?
Why is it 'por' and not 'para'? I thought por was used more to express a period of time.
Here por is used because there is an exchange...money for dessert. Also, your idea that para is used for time is faulty. Para is used for time when the time expresses a deadline...think destination. Por is used for time when expressing a period of time...think duration. Another case of "por" for exchange is when you say Gracias por venir, for example. You are basically exchanging your thanks for whatever you are thankful for.
My wife would always use “invitar” here instead of “pagar”. She is a native spanish speaker, so I’m going to say this needs to be an option.
Yes, invitar, when it comes to things like paying something like dinner, drinks, etc. is similar to the English verb "to treat"
¿Me invitas a postre?
Hmm...dessert...are you treating? Are you treating me to desert?
What we WOULDN'T say in English however it the literal... Are you treating me desert or Are you treating desert for me.