is this wrong?

So in my local woolies I saw a big yellow sign saying Boneless Pork Leg roast $7 a kg. When inspecting them they were still marked as $11 . So I put one in my trolley costing $26.
Went through checkout, paid for it, then went to the information desk and queried the price as I thought I saw a sign for $7 but it still charged me the normal price. Staff goes away and comes back, yep you are right, that means you will get it for free. Is it ok if we give you cash as we dont refund onto cards anymore.
So a $26 roast for free @watso thinking of you