If posBoss doesn't close off the transaction after it has successfully gone through the eftpos terminal, this means that the iPad has not received the "transaction success" message from the terminal over your network.
If this happens, you should be prompted with a message asking "Was this transaction successful?". Here you can select Yes to close off the transaction.
If this does not occur you can manually pay off the transaction to card by pressing and holding the card button (long press) . A small window will pop up. Here you can assign the transaction to Card, and bypass the eftpos unit.
If this is happening frequently, please let your eftpos provider know. Also, please check out the following article: