Total the extracted amounts.

    data = Dataset@Table[Association[
         "Sector" -> RandomChoice[{"Energy", "Env"}],
         "Supplier" -> StringTemplate["N``"]@RandomInteger[5],
         "Stage" -> 10*RandomInteger[5],
         "Amount" -> 1000*RandomInteger[200]
         ], 100];
    Total /@ GroupBy[data, #Supplier &, Map[#["Amount"] &]]