Regarding the error:

"The statement balance is incorrect ! The expected balance (0.00) is different than the computed one. (500.00) "

For this, you need to go to the Journal menu, open Cash journal record. Go to the Cash Register tab. If your Journal type would be 'Cash', You will be able to see one check box field Cash Control. You need to tick that check box and save it. By enabling that check box, You will be able to see Available coins. Now, retry to click on Close Cashbox button from Cash register form. It will work fine as expected.

Thanks, Priyesh Solanki

Thank you so much - I've got it.