Prevent ComboBox selection based on TextBox in Windows Application

Hello,

I have two combo boxes one of them displays a number of tanks and the other displays a number of raw_materials.

I want to make sure, if i picked a tank_1 and filled it with material_1 

If the user tries to pick material_2 to fill the tank_1 while the tank_1 has an amount from material_1, a warning message pops up, says that "you must not add another material till the tank is empty!."

Can anyone help me please?

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    GetRawTank1()
    GetRaw1()
End Sub
Private Sub GetRawTank1()
    ' CLEAR COMBOBOX
    cbxrawtank1.Items.Clear()
 
    ' QUERY raw_tanks TABLE
    SQL.RunQuery("SELECT tank_name FROM raw_tanks where tank_name = 'Raw_Tank_A'")
 
    ' IF RECORDS ARE FOUND, ADD THEM TO THE COMBOBOX
    If SQL.RecordCount > 0 Then
        For Each r As DataRow In SQL.SQLDS.Tables(0).Rows
            cbxrawtank1.Items.Add(r("tank_name"))
        Next
 
 
    ElseIf SQL.Exception <> "" Then
        MsgBox(SQL.Exception)
 
    End If
 
End Sub
Private Sub GetRaw1()
    ' CLEAR COMBOBOX
    cbxraw1.Items.Clear()
 
    ' QUERY raw_materials TABLE
    SQL.RunQuery("SELECT material_name FROM raw_materials")
 
    ' IF RECORDS ARE FOUND, ADD THEM TO THE COMBOBOX
    If SQL.RecordCount > 0 Then
        For Each r As DataRow In SQL.SQLDS.Tables(0).Rows
            cbxraw1.Items.Add(r("material_name"))
        Next
  
    ElseIf SQL.Exception <> "" Then
        MsgBox(SQL.Exception)
 
    End If 
End Sub