In the exit event for the field whose data you want copied to the master page:
form1.pageSet.Page1.Text1.rawValue = this.rawValue;
I'm making a couple of presumptions here: 1) The name of your master page is Page1 and the name of the text field is Text1, and 2) that you're handling the formatting elsewhere, or you don't care about it.
If the formatting does matter you can try this.
form1.pageSet.Page1.Text1.rawValue = this.formattedValue;