Maybe you can integrate something like this
Option Explicit
Dim ddIndex As Long
Dim ddDept As String
' Callback for DdropDown onAction
Sub DReportSelection(control As IRibbonControl, id As String, index As Integer)
ddIndex = index
End Sub
'Callback for Button onAction
Sub ExcelImport(control As IRibbonControl)
Macro4Department (ddIndex)
End Sub
Sub Macro4Department(i As Long)
Select Case i
Case 0
MsgBox "Department 1 is selected ", vbInformation
Case 1
MsgBox "Department 2 is selected ", vbInformation
Case 2
MsgBox "Department 3 is selected ", vbInformation
End Select
End Sub
<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui"><ribbon>
<tabs>
<tab id="CustomTab" label="TestLabel">
<group id="SampleGroup" label="BU Selection">
<dropDown id="DdropDown" label="Select Dept" onAction="DReportSelection">
<item id="item1" label="Depart 1" />
<item id="item2" label="Depart 2" />
<item id="item3" label="Depart 3" />
</dropDown>
</group >
<group id="SampleGroup1" label="TestLabel">
<button id="Button" label="Import Excel" imageMso="MicrosoftExcel" screentip="Import Data from Excel" size="large" onAction="ExcelImport" />
</group >
</tab>
</tabs>
</ribbon>
</customUI>