Cancel Ticket and Re-Purchase at Lower Price

I recently grabbed a one way reward in UC from LHR-JNB, and then purchased a Z return, intending to use the other leg later in the year. At the time of purchase I inteneded to purchase two seats but the agent misunderstood and I just bought one. In researching prices the same ticket is now about $1100 cheaper.
Could I cancel my cancel, get a refund, pay a penalty, then purchase two Z at the prevailing price.
Is there a rule against this?
Lastly, I've been through the VS website, but haven't found in the various terms, what any cancellation fee might be.
I can call the airline, but dont want to do that without some input from the guru's on whether what I'm doing is possible.
Thanks
Could I cancel my cancel, get a refund, pay a penalty, then purchase two Z at the prevailing price.
Is there a rule against this?
Lastly, I've been through the VS website, but haven't found in the various terms, what any cancellation fee might be.
I can call the airline, but dont want to do that without some input from the guru's on whether what I'm doing is possible.
Thanks