When you approve a bid, the system checks whether you have sufficient amount in the account. If so, then it is blocked for bloggers for 3 days. It is great, if a blogger writes a review during this time - the money has already been waiting for him. If he does not hurry, the sum becomes free, and you can spend it on the approval of another bid. Meanwhile, the first task is not deleted (since 3 days can be too short of a term for many bloggers), and the blogger, after all, completes his task. However, till that moment your account balance can be insufficient for paying his work. Then you will have a negative balance and with a negative balance any approvals of bids become blocked until you deposit funds at least to zero. If you do not want ever to have a negative balance, then cancel the unexecuted tasks after 3 days.