Looks like a weed seedling to me too. The plant also looks quite wet, which is not good. The plant looks probably too far gone to rescue, and I would have taken it back to the seller after deteriorating so rapidly. The colour of the leaves in the latest pic is really brown and it looks virtually dead to me. Sorry not to be optimistic.
It looks kind of like it has had a frost actually.
It looks kind of like it has had a frost actually.