Hi @Anonymous ,
Do you want to fill in the third field based on the first two fields, and if the third field display "Approval is required", you need to choose Approval for this item?
I've made a similar test for your reference:
1)insert a gallery, display two fields: recommended discount and requested discount
2)insert a label inside the gallery, set the label's Text:
If(ThisItem.'recommended discount '<ThisItem.'requested discount',"Approval required","")
3)insert a combo box inside the gallery to choose approver (approver is a person field in my list)
Set the combo box's Visible:
If(Label1.Text="Approval required",true,false)
Set the combo box's Items:
Choices(test1031.approver)
//test1031 is my list name, you could replace it
4)insert a button inside the gallery to submit the data of Approval and approver field.
Set the button's OnSelect:
Patch(test1031,ThisItem,{Approval:Label1.Text,approver:ComboBox1.Selected})
//if approver is a single person field
Patch(test1031,ThisItem,{Approval:Label1.Text,approver:ComboBox1.SelectedItems})
//if approver is a multiple person field

Best regards,