z.Add a(i, 1), CStr(a(i, 1))

For i = 1 To z.Count
        [A:C].AutoFilter 3, z(i)
        With x.SpecialCells(xlCellTypeVisible).Cells(1, 1)
            .Value = Application.Subtotal(9, x)
            .EntireRow.Hidden = True
        End With
        x.SpecialCells(xlCellTypeVisible).EntireRow.Delete
    Next

Option Explicit
Private Sub Merge()

    Const strOtherOutside As String = "Прочий внешний"
    Dim i1 As Integer 'количество строк на листе
    Dim j As Integer 'счётчик строк на листе
    Dim sngSubOtherOutside  As Single 'подытоги по контрагенту Прочий внешний
    
    'Склейка по контрагенту Прочий внешний
    With Worksheets("Данные")
        i1 = 0
        While .Cells(i1 + 1, 3).Value <> ""
            i1 = i1 + 1
        Wend
        sngSubOtherOutside = 0
        For j = i1 + 1 To 3 Step -1
            If .Cells(j, 3).Value = .Cells(j - 1, 3).Value Then
                If .Cells(j, 1).Value = strOtherOutside Then
                    sngSubOtherOutside = sngSubOtherOutside + .Cells(j, 2).Value
                    .Rows(j).Delete
                End If
            Else
                Rows(j).Insert shift:=xlDown
                .Cells(j, 1) = strOtherOutside
                .Cells(j, 2) = sngSubOtherOutside
                .Cells(j, 3) = .Cells(j - 1, 3)
            End If
        Next
    End With
    
End Sub