I had called the LH online help desk prior to cancellation to check if a full refund will be issued. They said yes and asked me to cancel online to get the refund faster.
I would never ever trust a call center agent saying something. You cannot prove anything. What was exactly said on the cancellation page. Did it say the Refund Total Amount would USD 6.800?
USD 6250 are at risk here.
I very much fear you lost that money (esp. because you have already received a refund).
To recover part of the money, maybe it could be a wise option to book a discount economy ticket to Chennai.