have you tried opening the dialog form with set range?
ie 
 frm.open("form")
     if not frm.id.setrange(idmaster) then
        error msg
     endif
 yesno=string(frm.wait())
ect, ect
 
I have used this in my app and it works like a charm.
There is a book out "Inside Paradox 5 for windows" that explains the whole 
process.
I hope this helps
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