Article 87 - Set-off by notice
(1) The right of set-off is exercised by notice to the other party.
(2) The notice must specify the obligations to which it relates.
(3) If the notice does not specify the obligation against which set-off is exercised, the other party may, within a reasonable time, declare to the first party the obligation to which set-off relates. If no such declaration is made, the set-off will relate to all the obligations proportionally.