If you have a copy of the contract you put forward you have a case, if you have an email you do not. The bank must sell to the highest bidder as long as the offer is conditionally reasonable, if your offer was subject to finance etc or long settlement, and the lesser offer was unconditional cash settlement, they would be able to explain their reasoning. But apples for apples with a genuine bid of similar conditions no doubt you have a case, taking on a bank and their well paid attorneys for damages could be a long expensive process. My advice find another property.