Can your offer be rescinded? Absolutely.
Is it likely that your offer will be rescinded? Not particularly, no. If the company has gone through a long interview process, determined that you're the best candidate for the job, extended an offer, negotiated a salary, started the process behind the scenes of getting you set up in all of their systems, etc. they're not likely to rescind the offer based on a small mistake. Of course, you might be unlucky and this mistake might disqualify you for this company or this position or this manager. Details like how long ago the collection was, what impact it has on your credit score, whether there are tons of other equally good candidates in line, whether the company invested a lot of time in interviewing you already or whether they're hiring a bunch of commission-only salespeople with the expectation that some will do well and some will get let go rather quickly will all play a role.
You would have been better served telling the employer earlier in the process about any negative information on your credit report given that you knew they would need to pull it as part of your background check. Not only does that give you the opportunity to explain the issue, that allows the hiring manager to tell you whether that would be an issue for the particular company and the particular position you're applying for. And it allows the hiring manager to diffuse the impact when the background check actually happens. Politically and psychologically, knowing in advance that there is a small issue is much easier to take than being surprised at the 11th hour.
If you haven't even completed the paperwork for the background check yet and you have a relationship with the hiring manager from your time as an intern, I'd drop him or her an email. Let the hiring manager know that you just became aware of an issue that's likely going to show up on your background check when they pull your credit. Give a brief explanation of the issue. Then you can ask the hiring manager whether you need to do anything special as part of filling out the background check paperwork in order to ensure that this doesn't become an issue. Most likely, the hiring manager will tell you not to worry about it. If it is something that is going to cause a problem in HR, though, the hiring manager can help you work through it.