calculation error vb.net textbox - vb.net

Question

I am working with inventory system, Transaction form I had mainly 2 text box, first 1 is avaqty its from database available qty, other textbox is avqty from user adding qty.
When user add qty avaqty textbox automatically change to
(avaqty) -(avqty) but as an example 10-10 result come in -1
My code is:
Public Class transaction
Dim objcon As New connectionclass
Dim qty As String
Public Function getqty() As String
If itemid.Text = "" Then
Else
Try
objcon.myconnection.Open()
Dim getqty1 As New OleDbCommand("select balance from avaqty where item_id=" & itemid.Text & "", objcon.myconnection)
Dim reader As OleDbDataReader = getqty1.ExecuteReader
While reader.Read
qty = reader("balance")
End While
objcon.myconnection.Close()
Catch ex As Exception
MsgBox(ex.Message)
End Try
End If
Return qty
End Function
Private Sub itemid_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles itemid.KeyPress
If Asc(e.KeyChar) = 13 Then
objcon.myconnection.Open()
Try
Dim getdata As New OleDbCommand("select*from itemtbl where item_id=" & itemid.Text & "", objcon.myconnection)
Dim reader As OleDbDataReader = getdata.ExecuteReader
While reader.Read
itemdesc.Text = reader("item_des").ToString
yom.Text = reader("unit_of_mesure").ToString
cost.Text = reader("item_cost").ToString
End While
Catch ex As Exception
MsgBox(ex.Message)
End Try
objcon.myconnection.Close()
End If
If getqty() = 0 Then
avaqty.Text = 0
Else
avaqty.Text = getqty()
End If
End Sub
Private Sub initem_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles initem.CheckedChanged
dataview.DataSource = GETDATA()
End Sub
Private Sub outitem_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles outitem.CheckedChanged
dataview.DataSource = GETDATA2()
End Sub
Private Sub avqty_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles avqty.TextChanged
Try
If avqty.Text = "" Then
avaqty.Text = getqty()
Else
avaqty.Text = CDbl(avaqty.Text - avqty.Text)
End If
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
Private Sub avaqty_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles avaqty.TextChanged
If avaqty.Text < 0 Then
MsgBox("not inough qty")
avqty.Clear()
End If
End Sub
End Class