This should set you in the right direction

HCL Notes/Domino 8.5 Forum (includes Notes Traveler)

Subject:

This code is untested:

Sub Click(Source As Button)
'Presuming the key value is unique, and that you can create a hidden view called "(keylookup)" with the key value in
'the first column, which is sorted

Dim session As New NotesSession
Dim ws As New notesuiworkspace
Dim uidoc As notesuidocument
Dim db As NotesDatabase
Dim view As NotesView
Dim doc As NotesDocument
Dim lookupkey As String
Dim invcount As Long

Set uidoc = ws.currentdocument
lookupkey = uidoc.FieldGetText("key")
Set db = session.CurrentDatabase
Set view = db.GetView("(keylookup)")
Set doc = view.GetDocumentByKey(lookupkey, True)
invcount = doc.invcount(0)
invcount = invcount + Clng(uidoc.FieldGetText("newamount"))
doc.invcount = invcount
Call doc.Save(True, True)

End Sub

Feedback response number WEBB8TMDFB created by ~Justin Chugeromanli on 04/23/2012