Yes, both of those sentences are correct and grammatically sound.
"Give me my money back": This is a common way to ask someone to return the money they owe you. The sentence is direct and straightforward, indicating that you want the person to give you the money they borrowed or owe you.
"You have to give me the £20 back": This sentence also conveys the same idea but includes the specific amount of money (£20) that the person needs to return to you. It emphasizes the amount and makes the request more precise.
Both sentences are appropriate in different contexts, depending on how specific you want to be in your request. The first sentence is more general and can be used for any amount of money, while the second sentence is more particular and pertains to a specific amount (£20).