I'm not American, but have visited the US a couple of times. I think in one of those trips I got this $100 bill:
As you can see, it has some dark stains, but it didn't bother me at the time and I saved it for future trips.
Recently, I tried to use this bill in Argentina (a country which has its own currency, but is receptive to dollars), but the clerk in the exchange didn't accept it because of the stains.
My next trip is to the US and I wonder if it's OK to use that bill in there. Is it?
One thing that makes me concerned: One of the tests for whether a bill is counterfeit used to be a marker which didn't react with real US currency but became visible when applied to other printed products. So you just might have been stuck with a counterfeit.
But there are a fair number of ways to check if a US bill is counterfeit, and since that's using the newer design it should have most or all of them: embedded "thread", a small area of ink that changes color depending on the angle you look at it from, microprinting. A websearch should find a page which shows you where to look for these, and if you bill has them then the mark is probably not reason to worry. If it doesn't, you have an expensive but illegal souvenir.
If it is counterfeit and you try to use or exchange it, you could be in for some very uncomfortable conversations with police until they were sure you weren't trying to pass it deliberately, and it would be taken from you. And no, you wouldn't get anything to replace it. So take the time to check. And knowing what to look for will help you protect yourself from the risk of USD counterfeits.
So one suggestion is to ask a bank if the bill is legit and, if so, exchange it there. Taking damaged bills out of circulation (along with releasing new bills into circulation) is one of the services banks provide, so it's not an unreasonable request.
Stores in the US may refuse to accept it not because of the stains, but because of the value. It is very rare for Americans to use the higher denomination bills in day-to-day shopping, and many stores refuse to accept $50 and $100 bills altogether. Incidentally, I've also heard of stores refusing to accept $2 bills and $1 coins... because they didn't know those exist and thought it was a forgery.
As mentioned in the comment, you can always exchange it in a bank. Stores with fraud detection machines may agree to accept it if the fraud prevention components of the bill are not obstructed by the stains.
I've seen worse, but on smaller value bills (some $1s and $5s in bad shape are pretty common).
