once off data bought. don't allow me to use it

Have a contract with data included. It was used up one month before new inclusive was due. I bought a once-off data bundle. With real, non-expiry money. Telkom does not allow me to use the once-off data while I have inclusive data. And now they are taking it back, because I did not use it before the 'expiry'. To me it feels like they are *****ing from me! They feel they can do it because it's (somewhere) in the T&C's. I find this unacceptable behaviour. (btw. They also 'cant' transfer the data to my other number.) So I have no way to use a product which I paid extra for, I have no recourse, and they have T&C's, they have my money, and on the 1st they will have the rest of my once-off data bundle. How is that fair business practise?? (ref ***3)