Restrict agency segment in line based on BU sel

Summary:

On Purchase Orders, is it possible or are there any workarounds to restrict the COA department segment in the line level distributions, to only be child values of the BU entered in the header level?

Example: Header BU = 123; Distribution Departments allowed should be 123111, 123112, 123123. Agency 678123 should not be allowed to be selected.

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