I ordered this as a seedling at the beginning of October. I planted it in half peat moss for acidity and half cactus soil because it needs a very low nutrient soil. I put in in the sun for a few hours a day and kept it inside where it can stay warm. I have also sprayed it with Neem oil just in case. It was fine for a while. Then it developed discolored patches, and the uppermost leaf was curling up, so I figured I was overwatering it. It was pretty soaked. So I uprooted it, removed the soil that was on the roots, and replanted it in the same soil. I put a layer of dry cactus soil in it to absorb the excess moisture. It was a warm day today and I left it in the sun for longer than normal. When I got home it looked withered. I have it inside now and gave it a tiny bit of water just over the roots. Is this just sunburn, or is the presumed overwatering killing it? All Ibwant to do is keep it alive long enough to fruit. I have no intention of planting it in the ground as I live in an apartment. They're supposed to do well in pots and love sunlight with infrequent waterings. Oh, I'm in Southern CA, so it's cooler than normal but it doesn't get cold here. The humidity has been high lately, so ambient dryness might not be a factor here.

Your seedling (because that's what it is) hasn't been hardened properly and is now heavily stressed.

I am not overtly familiar with Miracle berries, but like baobab and other exotics a few pop up now and then and they are usually tiny and don't appear to have been hardened properly.
The hardening process in these exotics, when grown outside their native areas, is to expose them slowly to the different growing conditions so they can adapt. For example in high moisture plants like this one they should be covered with either a clear plastic bag or a glass jar to keep high levels of air moisture and be progressively exposed to our drier climates, with regular misting providing high levels of moisture.
Light is another factor: standard practice is to grow these seedlings inside and in indirect light until they are large enough, then they can be progressively exposed to direct sunlight, all the while providing them with adequate levels of moisture.

Finally what you did. Snake o..., I mean neem oil + direct sunlight often results in leaf scorch. We've seen several cases on this board over the past 4-5 months.
You uprooted it, meaning you stressed it and damaged its tiny roots. That's why it wilted when left in the sun.

What you should do not? I advise you find a nice spot inside the house with plenty of indirect light and put your seedling there. Put a clear plastic bag over it to keep moisture levels high and remove it once a day to wipe the excess moisture and to avoid algal growths.
Do nothing else: allow the seedling to recover and rebuild its strength then, if it survives, you can start the hardening process anew.

Totally agree with El Pollo DIablo.
You are the parent of a tiny fragile baby plant. Good but indirect light and the moisture tent will support its little internal 'factory' of photosynthesis so it can grow.