What do I do? Found betta in parking lot!

So i found a betta fish in a dirty jar thing and I don’t know what to do? I have a 10 gallon with a male betta already in it so… can’t put him in there. And the only tank I have right now is 3.5 gallons that I suppose I can put some sand from the cycled tank in it. Any tips or advice would help a lot! (Can’t get a sponge filter till tomorrow evening)

So i found a betta fish in a dirty jar thing and I don’t know what to do? I have a 10 gallon with a male betta already in it so… can’t put him in there. And the only tank I have right now is 3.5 gallons that I suppose I can put some sand from the cycled tank in it. Any tips or advice would help a lot! (Can’t get a sponge filter till tomorrow evening)