Why not gate agent looked at the payment when he issue BP
YES. UA issued the BP. There would have been time to "work it out" at that point rather than when a flight needs to depart. UA is at fault here by issuing the BP without seeing if payment had been made in time to correct the error. But, I suspect IDB doesn't apply since IDB is strictly only for overbooked flights. I don't think there is a provision for cases such as this.