Betta Fish: “Gifted” a betta—is it alright to keep him in this cup for the next day or two until I get a bigger tank?

Hello everyone! I’m currently a teenager living with my family. A few hours ago, my younger sibling bought a betta fish from petsmart on a whim, and we have nothing prepared. They’re not responsible enough to take care of it alone, so I’ve kind of adopted him.

He is currently stuck in the cup. A 5 gallon tank is on the way (don’t have the room for a 10 unfortunately) but it will take two days to get here.

Pictured in the second slide is a slightly larger tank (microwave for scale), but I am hesitant to move him in due to worries about acclimation (I have no water conditioner, and I’d be moving him again the day after anyway).

Is it alright to keep him in the cup, or should I try to move him into the “larger” tank straight away? Is there anything else I should do in the time being?

Thank you so much!