Some people (I'm one of them) have a nervous tic that exhibits as scratching, picking, worrying little skin irregularities with the fingers, even pulling out hairs. Because I have typically sensitive "redhead" skin (this is more common, I hear, among easily-irritated fair skin), my fingers seem to believe they always have important work to do.
It takes tremendous dedication to make myself stop scratching when I get any small skin injury. Keeping busy, doing tasks that involve my hands, and engaging in lots of physical activity are helpful. Boredom, or being unable to move around freely, make the problem worse. You might experiment with dance or other physical exercise in the late afternoon or before bedtime so your darling daughter goes to bed really tired, and see if that helps.
I also tend to pick/scratch more when I feel nervous or anxious (I have been dealing with an anxiety disorder for many years). While I'm old enough now to deal with my own reactions to other people, your daughter isn't. So please try to avoid seeming anxious about her habit, which could make her more likely to do it. When you treat the owie or talk to her about it, try to stay calm and cheerful. Perhaps a touch of humor would be good for you both.
A note on BRIBES:
Bribing is probably never a good idea. You can generally avoid using the phrase, "IF you will _____, then I will _____." You can still reward and positively reinforce with a change of language. Simply substitute, "WHEN you do _____, then I will _____." There is less implication that the child has leeway NOT to behave in the way you need, and a more positive suggestion that your desired behavior will take place. This is especially important if the behavior you desire will have to happen anyway, like taking medicine or getting into her car seat to go home. This approach leaves you free to reward your child even if she fights you while getting her to do what is needed.
This worked wonderfully with my daughter. My expectations remained clear and non-negotiable, and there was plenty of room for her to be a little kid.