Think about this. Let's assume that you landed a job tomorrow making four or five times what you make right now. Great, right? Sure, it might boost your ego, but don't forget that you're (psychologically) addicted to heroin and when that first paycheck hits, you're going to have a really hard time not justifying a little splurge. I'm not saying that it necessarily would, but it's believable that you could build up a good little gram-a-day habit if the money was seemingly unlimited like that. I blew through money so unbelievably fast when it was readily available (and my tolerance skyrocketed, I started getting sicker and sicker in the mornings, and then one day the money ran out and I had to learn how to spend money that wasn't mine, and then it all got even worse...). And with that habit and it's collateral damage in motion, even if you did maintain the job, you probably wouldn't be profiting as much as if you stayed clean and stacked your pennies at your current job. So would it really be that different? It would be the same as now, only amplified.
Of course you're going to tell me that conditioned on you actually getting that much better job, you'd be much better able to put heroin behind you. All I can say to that is "well I hope that you can prove me right on that one day!" Heroin is a really powerful addiction and it does not discriminate. Walking away from that was the hardest thing I've ever done (and I've done a lot in my life), and it's definitely not something that can be "switched off" upon something like a promotion. Getting clean now, though, will (1) make staying clean once you get that better job with the fatter paychecks easier and (2) will better give you the chance of getting that better job at all.
If that does not work, then as a friend I just don't want to see you go through some of the shit I did. Any of it, really, from true dopesickness to legal troubles to doing things you never thought you'd do. You're way too good of a person for any of that.