This guy might not be honest. At this low miles, and in the absence of any other problems coming from the trans? I’d dump this guy like a bad habit. ALL used transmission fluid is going to have dirt even if it only has a couple hundred miles on it. This dirt could be clutch material, wear from the gear teeth, a combination of lots of other things. So long as the vehicle hasn’t been in water or some such slightly dirty trans oil is completely normal and not necessarily indicative of any problems. Sounds to me like this guy is trying to sell you unnecessary repairs, he wouldn’t be the first. But thing is he could not be a thief he could just be full of wrong ideas and maybe he really does believe the trans is wearing out. I was in automotive for 30 years and the number of mechanics I met who were more or less skilled, but full of wrong information, never ceased to amaze me. So I’d take the high road and not condemn the guy for being full of it but I wouldn’t take my car there again either.
I’d just drive the car, check the trans fluid once a month, and forget about it. If he was right, the trans will conk out and then and only then you have to pay up. But I don’t think this is very likely, given the info you told us, if it still worries you there are companies that can take a sample of your trans fluid and do an analysis. They will read the PPM or percentage of contaminants and tell you if its excessive or not. Also the most important thing, make sure they used the right trans fluid and not just what they had in their flushing machine that day! I used to work in a shop that had one of these machines, refilling fluid once its in the reservoir is not easy and a lot of sloppy mechanics might not bother to use the right fluid (I saw that done too more then once). The dipstick or owners manual tells you which is correct and most of them do not interchange, so long as the fluid is “compatible” (according to the fluid maker) that’ll do. If they used the wrong fluid you will soon know it since your trans might start slamming into gear and slipping sometimes, if that happens have them drain it out and GIVE them a couple gallons of the proper fluid to use, ask them to use that only, (no reason they wouldn’t do it unless you didn’t give them enough fluid for the job i guess). But i change my fluid every 30,000 miles and have good luck with that in all the 10 cars i’ve owned so far.