If you wish to test in sandbox mode how a failed or returned  bank transfer payment works, here is an easy way to test it. It takes seconds to return.

Got to Payments > Create Payment > Add Payment > Send Payment

Failed payments:

Simply add the text "!failed" (without the quotation marks) into the Memo field of a payment in sandbox. The payment will automatically fail. Keep in mind this command has to be the first characters of the memo.

Returned payments:

Add the text "!returned" (without the quotation marks) into the Memo field of a payment in sandbox. The payment will automatically return. Keep in mind this command has to be the first characters of the memo.

NOTE: Using this method to test failed/returned payments in your sandboxed account will only work with bank transfer payments.