Most of the time a distribution from a 529 plan cannot be “undone” by putting the money back into the plan. The plan administrator will simply treat the replacement money as a new contribution, and the Form 1099-Q issued at the end of the year will still show the original distribution amount. However, if the beneficiary's school returns money previously taken as a qualified withdrawal to you, such as a tuition reimbursement, you can recontribute those funds back into the account anytime within 60 days of the refund without any tax penalties.
