"I pay today."
If you're going to create a sentence with は and が, you're either going to need to italicise the word you want emphasised, or accept the particles in either combination.
Without context, we have to guess. And for new learners, there's a likelihood of internalising the pattern you've given as the only correct response, and the other one as incorrect (when, both grammatically and in context, this isn't the case).
I think it is meant like this:
"As for me, I pay today."
This sounds like only you pay today. Maybe someone else pays tomorrow.
As you are doing the thing, you are the subject (marked with "ga"), but you are not the topic (marked with "wa") because the topic is the general gist, the most important thing - here, the fact that you are paying is less significant than the fact that you are doing it today.
So "wa" for "today" and "ga" for I.