under the law the price you see is the price you are legally allowed to pay until changed by the seller. wether a mistake or not no company can change the price after and then say u owe us the rest.
if players of that rank bought those things at the rank for that price they are protected by law not governed by grees tos.its gree staffs error and now gree have fixed their error and corrected the price of buying said items.
so for those players who earned that rank they should be allowed to keep them.
as for players selling... thats something else.
if i was to keep playing this bugged up trash. at a guess my game would take a year plus to reach their level of rank so they should not be penalised for gree fkup. imo.