The issued bill is the bill that has been settled. What you are looking at is the transaction details. The + sign means that the money was used by you, not for repayment, and the entire amount of the current bill is due. It is 0 yuan, which means that you do not need to repay the current bill. This may be because you paid more money on the previous bill and had a balance, which was offset against the new debt in this bill.