It could be just a mechanical damage that healed over - it could have been 'bumped' while handling; there could have been some pest bites healed over. Unless it looks soft and mushy, it is probably nothing to worry about.

The stem in second photo seems to be ok too, some older or damaged leaves fell off and you see more of the stalk.

I can't tell very well from the photo, but roots seem to be above soil level? - could be just my imagination.

Did you put it into bigger pot? I think I am seeing all of the original (peaty) mix in there intact - when re-potting, it is best to get rid of all soil around roit ball, and pot into well draining mix.

Thanks, I've had this plant about three weeks in a 3" square plastic pot and just move it to the breathable Terra cotta one. When I got it from a greenhouse it was yellowed from being in a office of a ex-coworker for weeks. I'll be shaking more of that old soil off to add my cactus mix. Great news!

I agree with Rina that it could be just mechanical damage. Watch and see if it spreads over the weeks.

It is a Corpuscularia lehmannii, a species that likes lots of light and gets quite wrinkly when it needs to be watered: Corpuscularia lehmannii Info