After you file

 I need to fix my boyfriend's refund of a misfiling error now he OWES $300. I know I can fix this with 1040x, but not sure of what to do first. Pay $300 and request 1040x documents? Does he have to pay $300 (I will for him if need be) an error due to me misplacing the exact document he needed to send in and we were worried why he wasn't getting his return. Well I'm too crazy to remember where it went or maybe even thrown out it's been such chaos as I'm sure I'm not alone, can I request the doc I misplaced ? Or just amend right away.