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>