What can they do? Just want to know the consequences before I make my decision to return.
If you have that kind of thinking, good luck in life.
You know the money isn't yours, you did no work or provided a service to warrant the payment, yet you want to hold on to it, play the wait and see game.
It is not as if you found the money left in a bag somewhere with no form of identification.
If that was the case, then nobody will say you are dishonest.
Here, you clearly know the money is undeserved.
The money could be meant for someone who has actually worked for it - you are now probably holding onto someone's salary.
You want to wait and see - they can also wait and see - they can freeze your bank account to prevent the transfer/withdrawal - while they investigate.
You like your bank account to be frozen? No more ATM withdrawals, no more cedit/debit card access?