We have a commercial customer who is accepting cash ($20's) payments for fuel delivery to a Mexican National. Our customer is filing 8300 reports with FinCEN and we have matched them up to our CTR filings.

Should we also file a SAR on the Mexican National for the LARGE cash payments in $20 denominations, even though our customer is reporting him on the 8300 report?

Add your customer to it also for potentially aiding and abetting money laundering.

We have a commercial customer who is accepting cash ($20's) payments for fuel delivery to a Mexican National.

How much? how often? is this in line with fuel costs in your area?
Personal account or business?
Resident alien or living abroad?
is the money coming from or leaving the US?

I do not believe that we at bol have enough information.

If a customer deposits a reportable amount of cash and we believe it's suspicious based on any criteria, including the amount alone, we're going to file a SAR. The fact that we filed a CTR already doesn't excuse us from SAR filing. I'm guessing your customer isn't able to or required to file SARs, and I'm guessing they aren't inclined to call up their local law enforcement to report suspected money laundering on their customer.

Whatever your decision, you need to document how you came to it. If you think it's suspicious, file. You can decide if the 8300 form helps mitigate whether your customer is a subject or not, but even if you decide they aren't you should still consider whether a SAR is warranted with the buyer as a subject or no subject identified.