It sounds as if the person creating the case is entering the disputed amount as $300. The person who investigates the case sees a credit for $700, and thinks you have received more than you requested.
I would enter the dispute, in writing as required by law, for the full amount of $1000.
The first one or two cases, they made the dispute $300. They wouldn't let us do anything else even though we protested. The last two cases, which yes, we did have to fax things several times (they lost them, shocking), we did dispute the full $1,000 and explained we did receive $700, but that isn't a full-refund as shown by the documentation attached. No one from Barclay has even contacted the vendor, and this has been open since July!
My husband called yesterday and the last case (#4 I think) was closed, but #3 is still open they claim...and are continuing to investigate. We always knew Amex had the best CS, but really are still shocked at just how bad Barclaycard is with disputes. I've had great luck with Citi, so we'll stick to Amex or Citi in the event we use a new website/seller.
PS everyone stay away from despegar.com obviously, and shame on tripadvisor for showing them as the lowest rate. Live and learn, we wanted to lock in the lowest rate for a wedding in Cancun, but then the wedding planner had a lower rate a month later so we jumped on that instead. The Barclaycard Arrival came with $400 of free credits so I guess we're technically still ahead by a small amount...