It is your right to demand back the old document signed, to prevent any unforseen circumstances, they can't simply 'buang' the document <-- lousy excuse and irresponsible. Even want to destroy, it must destroy in front of the customer.

You can complain to relevant authority and get your own lawyer to sort it out. Some money spent is worth if later found out troublemsome in the future.

*Move to property section

Just some minor hassle.

Be positive. Take it as learning experience. You might learn something in the process of dealing with it.