Are you the agent or the owner?

Bank wouldn't know that the price had been marked up.

Only the buyer, seller and SPA lawyer will know as usually the lawyer will prepare an additional 'variation' letter to indicate that the owner is giving some discount etc. in order for both parties to agree on the 'actual' selling price.

If you are the owner, be prepared to pay extra on legal fees as it will be based on the marked-up SPA price (but usually you can ask the buyer to absorb the differential amount).