A man wants to pay me off to leave my girlfriend

I've been dating this lady for about two years now and she has been pressuring me to propose. We are both 29 and sincerely I am not ready to get married but I do like her and want to continue the relationship but amazingly about a week ago I got a call from a man offering me N500, 000 to end my relationship with her. The man is my girlfriend's ex-boyfriend who went to America 7 years ago. He's back in Nigeria now and wants to marry her but she told him she's in a relationship with me and can't leave me. I think he believes he can get her if I'm out of the picture hence the offer. I really like this lady but N500, 000 is a lot of money, I'm tempted to take it, especially since I am not ready to settle down. If I take it would that make me a bad person? I want my girlfriend to be happy and getting married would make her happy, so I would be doing her a favour by letting her go I think.