Let that be straight.

To state something clearly and honestly so that there is no confusion or misunderstanding.

Usually signifies the end of a conversation.

“I’m not interested in dating you, so let that be straight.”

“You owe me $50 for the pizza we ordered last week, let that be straight.”

You are a friend and you want to clear up a misunderstanding with another friend. You say: “I’m sorry if I hurt your feelings, but I was just joking. Let that be straight.”