You can only take this up directly with the store. You will need to provide details and a copy of the purchase receipt you hold.
Are you also saying that you physically returned the iPad but you do not have the receipt for that return? That may be hard to prove.
No-one in the Forums can do more than advise you as I have. The situation has to be negotiated with the store management.
Let us know how you get on. Hope this helps.
Seventy one is correct, you need to take this up with the manager of the store.
Ultimately, if the guy who handled the return at the store didn't enter it into the system correctly, or if the paperwork was somehow lost, or anything else along those lines, your refund receipt is the only thing you have proving that they owe you money. If you have lost that receipt, it's quite possible that you will have lost the money that it represents.
Go to the store. Talk to the manager. Try to recall which person at the store handled the return, and see if they remember you. Be polite but insistent, and remember that without the receipt, you are depending on those people to help you out.