You said you already do water by the chopstick method.
Tom is right that ficus is famous for easily dropping its leaves when conditions change. But since I have a full sized ficus, which did some of that this fall when it was brought indoors, I can say that they just fall. They may be green when they fall or they may yellow some, but they aren't brown and aren't curled. I think browning and curling is something else.
It sounds diseased. And since you say you only had this tree for two days, it sounds like it was already diseased when you bought it, just not as obvious yet.
I did find this:
In root rot, leaves may turn brown-to-black and curl up, eventually falling off.
https://homeguides.sfgate.com/fiddleleaf ... 92765.html
Are your leaves curling up or curling under? that can be diagnostic.
Root rot is a fungal disease, that occurs when the soil stays too moist. You say you are watering by the chopstick method. If you are doing that correctly, the soil will not stay moist all the time, it will be allowed to dry out some in between. But having only had it two days, you don't know how it was treated before. At this point, I think I would pull it out of the pot to check. As long as it is not wired in, you can do it gently. But if your tree might really have root rot, you need to know this.
If there is root rot, the affected roots will smell rotten and be brown and mushy:
There is no effective chemical treatment for these diseases. Discovery of fungal infection and root rot should be treated immediately, whatever time of year it is found. Trees should be lifted from their pots and ALL affected roots and woody growth should be removed back to healthy wood. Hopefully enough live tissue will remain for the live roots to regenerate and for the tree to survive. All infected soil should be burnt or binned along with any infected growth that is removed. The tree's pot must be sterilised with a disinfectant before repotting to avoid re-infection. Use of a very free-draining open soil mix (preferably with no organic matter) will make conditions for any remaining fungal spores very difficult.
What is the soil like that your tree came with? Cheap bonsai are often in regular potting soil or even field dirt. As this notes real bonsai soil is very mineral and very loose and free draining. If you take it out of the pot, you will need to have some bonsai soil to repot it with.
As tom noted we have a whole bonsai learning section here: https://www.helpfulgardener.com/bonsai/
with article on watering, soil, etc.
You said you realize you need a larger pot. Perhaps, I don't know anything about the size of your tree or your pot, but the art of bonsai is about maintaining trees in very tiny pots. I don't think the pot size is the difficulty here, although it may be that the tree has been in that pot a long time and needs root pruning to stay in it. The pot could be too crowded with roots, which leaves little room for soil, water, etc.
For future reference, a grocery store is not a good place to buy bonsai. They would rarely have appropriate conditions for maintaining them and they certainly would not have any people who know how to care for them. I also discourage people from buying bonsai from street vendors and pop up stands in malls that will not be there when you go back. If you don't want to be disappointed with a tree that dies, go to a bonsai dealer who has some reputation to maintain and will be there when you go back. It will cost more, but compared to having a tree that is dead in two days, it is worth it.
PS welcome to the Forum! Glad you found us. If you continue posting, please do put it in your profile where you are located. That is important for almost any gardening question, even indoors.