If the contract allowed you to live there rent free, I would think it would have stated "R 0".
I read "R N/A" as you won't be renting.
However, since you got the keys from them, and have been living there since Sept, it could be argued that your interpretation of not having to pay rent is more likely correct.
Have you asked the lawyers / estate agent?
What's holding up the sale?
Edit: They can't unilaterally change the agreement. You will have to agree to pay occupational rent. They could possibly also then try to evict you.
Occupational rent is not unfair though, and it's usually less than the bond payment + rates & taxes etc. You will have room to negotiate since you are living there at the moment.