Legal Question in Business Law in California

Ordinarily, yes. I can imagine circumstances in which it couldn't, but they're unusual.

The general rule regarding legal duty to reimburse unintentional overpayments is as follows: The party receiving an overpayment has a legal duty to refund the excess amount to the paying party UNLESS two things are true: (1) neither party was aware, nor should have been aware, at the time of the overpayment, that it was indeed an overpayment; and (2) the party who received the overpayment has materially changed his/her/its position in justified reliance upon his/her/its belief that the payment was correct.

Unless both conditions are met, the overpayment should be returned. A "material change in position" doesn't mean you just took the payment and deposited it in your checking account. A material change in position would be something like using the money to pay your creditors AND not having ample other sources of money to replenish your wallet.

OF course.

generally yes