Bank B is under no obligation to return the funds, whether they are available in the account or not. If the funds are available, bank B may choose to return, but only if a letter of indemnification is provided by bank A.
whether funds are returned or not, Bank B does need to look at this to see if potentially they have customer that is knowingly or unknowingly acting as a mule for fraudulent transactions. Turning a blind eye is not an option in this day and age.
Providing alternative truths since the invention of time