Absence record formula question/suggestion/ideas please

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Dear Oracle Customer Members,

We have two absence plans: Calendar Days, and Days (defined in the UOM).

Now, we are looking to create a fast formula to increase the security of our absence records management by the employees:

The expected result with the mentioned formula is the following:

• One employee selects by first time it absence plan.
  • Select the dates
  • Save the record
• Now the system prevents that the employee selects another one.

 

In other words: If an employee selects one of the two plans for the first time, and save a record, won't be able then to select the other plan.