Values allocation in 4 textboxes - vb.net

Question

I have 4 textboxes and one label on my form, 4 textboxes represent 4 payment types (cash, card, bank transfer and other payment method).
In my label i have summary value which i increment when i'm adding new item to receipt.
Textbox names are: cashTbox cardTbox bankTbox otherTbox
Label name is: summaryLbl
By default cashTextbox.text is equal to summ.content and every time I add an item to database i increment summaryLbl + itemPrice (item price from database).
I have tried making a function which will allow me to subtract from cashTboxto other payment types without allowing cashTbox to go to negative value.
My code is as follows:
Private Sub cashTbox_LostKeyboardFocus(sender As Object, e As KeyboardFocusChangedEventArgs) Handles cashTbox.LostKeyboardFocus
If sender.IsFocused = True And sender.text.length > 0 Then
If calculateP(sender) = True Then
End If
End If
End Sub
Dim sumP as decimal = summaryLbl.content
Dim cashP as decimal = cashTbox.text
Dim cardP as decimal = cardTbox.text
Dim bankP as decimal = bankTbox.text
Dim otherP as decimal = otherTbox.text
Public Function calculateP(ByVal s As Object)
Try
If String.IsNullOrEmpty(s.text) Then
s.text = "0,00"
Else
s.text = Decimal.Parse(s.text)
End If
If s Is cashTbox Then
ElseIf s Is cardTbox Then
cardP = s.text
cashTbox.Text = sumP - (cardP + bankP + otherP )
ElseIf s Is bankTbox Then
bankP = s.text
cashTbox.Text = sumP - (cardP + bankP + otherP )
ElseIf s Is otherTbox Then
otherP = s.text
cashTbox.Text = sumP - (cardP + bankP + otherP )
End If
cashTbox.Text = sumP - (cardP + bankP + otherP )
Catch ex As Exception
End Try
setValues()
Return True
End Function
Public Function setValues()
summaryLbl.content = sumP
cashTbox.text = cashP
cardTbox.text = cardP
bankTbox.text = bankP
otherTbox.text = otherP
End Function