This looks a lot like a tiny maple seedling in it's first year, note the cotyledons - the seed leaves, the (in this case long & tongue-shaped) leaves that appear before the true leaves - which are still present. The pictures are a bit blurry, so I can't be absolutely sure but I've seen dozens of these under my Granny's Japanese Maple.
Potting up temporarily in potting soil instead of garden soil would have been my choice, but don't disturb it again. Get it out of the direct sun asap1: It will loose too much water and the roots are not established yet to "drink" enough. Water consistently, but don't keep it wet - humid will do. Water from the top, at least until it's established, because you can better control where the water goes. If you water from below, there is a chance that the water doesn't reach the roots as intended and you can't look into the pot... It should perk up soon. Acers can be a bit fussy when transplanted as small seedlings, but generally speaking they are robust little things.
Choose a final place for it and plant it there either in a few weeks when you see it's stable again (preferably during an overcast, rainy phase, avoid summer heat) or next few years. But remember that if you keep it in a pot longer, it will need constant care like watering. But it's perfectly doable, I know a Japanese maple that's been in its pot for at least ten or fifteen years now - doesn't grow too much, but appears quite happy. If you want it to grow to its full height, it needs to go in the garden, though.
If it's really a baby-maple, it may look like this in two years:
1 This is a good rule of thumb for all transplanted plants: When you root them up, they need time to establish a good contact between roots and soil again, which means water supply will be hampered for a while. You want to minimize water loss, so put pots in a shady place for a few days or choose overcast days with potential rainfall for digging up and planting plants in the garden. Those hot sunny days are meant to water the garden, smell the roses and enjoy the results of your hard work from a lawn chair ;-)