I had a friend visit a doctor and he issued him a receipt for payment, written on his official little "post-it style" prescription slips. We were laughing about it, but is it possible for doctors to use presc slips this way?
There is generally no prescribed form for a receipt
A receipt is simply evidence that money changed hands and it can be written on anything. Or recorded in a different way, say an audio file saying “I, Dave, received $100 from Jim.
There may be taxation rules about the information that needs to be in a receipt but generally not about what it’s written on.