The Products appearing on the Sub-Form are related to the Supplier record on the main form. The products value summary is calculated in the footer of the sub-form.

You may create a command button on the main form and on the button click event procedure you may write an expression to add the total to the supplier amount field of the current record with the following expression:

Private Sub Command0_Click()
       me![SupplierAmount] = me![SupplierAmount] + me![productsAmount]
End Sub

Make changes in the above code for proper object references.