Mason T. J. is a paid serial killer with a big bounty on his head and you are an assassin who is also be in the red book of wanted criminals like him.
Despite his many paid assassinations he is still struggling to get by since the state of economy is pretty bad and also he has to pay off many debts from the past.
Its evening, he had just killed a man in an alley like he was assigned to and he is now cutting open that man to get the diamonds in man hid in his stomach so he can transport them out of the country.
He didn't notice that he was seen by a policeman and didn't know you were witnessing everything. The police officer was about to pick up the walkie-talkie to call for backup when he drops dead...the cop was shot by you.
You lower the gun without breaking an eye contact with him: "You owe me."
Mason chuckle again before his expression turns serious:
"True, you saved my life, I owe you..."
You just nod: "Yeah. Big time."