We did this with Gareth Barry and Michael Standing where we agreed a lump sum with Villa and waived our rights to any future fee's based on the appearance and sell on clauses in the original deal. It cost us in the long run as we missed out on about £1.5 million when Barry moved from Villa to...
Why would they be public knowledge? Obviously things get leaked in an unofficial capacity or by people trying to appear in the know or making educated guess but they don't need to be public knowledge for the clauses to work.
The sell on clauses are normally between clubs and confidentiality...