ESPP -Plan 423- Unused fund Carry over or Refund to employee?

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We have an Employee Stock Purchase Plan (ESPP) that includes two offering periods each year, with purchase dates at the end of June and December.

My question is whether we should carry over any unused funds to the next purchase period or refund them to the employee. I would prefer to carry over the excess funds to the next period. However, what happens if an employee withdraws from the plan or chooses not to contribute in the next offering period?

Are there any potential issues with carrying over the funds in that case?

 

Thank you.