A Walmart greeter has no legal powers or authority to force you to show them your receipt so you can do just like I do and politely decline them looking at your receipt and walk right out the door.
As long as you have that receipt in hand it's unlikely that you've stolen things and a shoplifter will not have receipts with them with the things they walk out the door with.
All the Walmart Greeter can do to you is call the police on you but it's unlikely they will do so unless they actually think you stole something and usually that will only happen if you walk out the door with a large or expensive item.
Otherwise the police will likely not even come over them calling about a person walking out without showing a receipt.