Just bought used 2004 volvo xc 90 115k miles and tans is slipping

So, I bought a 04’ volvo xc90 for $10k and I thought it was a good deal. The blue book on this car is around $15,9k. After putting a deposit on an 03’ saab 93 I stopped by a local car dealership on the way home from work just to see what he was asking for two cars that caught my eye on the lot. A 00’ BMW 3 series with a 100k which he wanted $10K for I predicted that price before he even said it. Then I told him I had just put a deposit on an 03’ saab with 100k for $6300. I then asked how much for the volvo and he said he had it listed at $11,9K but that he would take $10K. After going home looking at the blue book and talking with my banker I decided to get the volvo. I was robbed of my deposit on the saab and now I’m looking at $4k for a new transmission on the volvo. The car has a 30 day warranty on the drive train, and I made sure to put around 2800 miles on the car in that time when the urgent transmission light came on driving home from work. The car was reving at 3k rpms and going around 10-15 mph from take off. Finally, a loud clunk and the transmission shifted. Then car seemed to be underperforming the rest of the way home. I got the car back today from the mechanic who put a filter screen and replaced old trans fluid with synthetic. On the way home taking off from a stop light the car revved highly then finally clunked abruptly in to gear. I called the mechanic back and he said that it needs a new transmission and that what he was doing was simply trying to prolong it. The guy I bought the car from was trying to tell me to get an extended warranty, but I called him today after this incident happened and he did not seem pleased. So, I’m trying to find a new transmission for this thing and i was wondering if you guys had any suggestions about getting a transmission for a reasonable price and so the problem will not reoccur. I kinda feel for the guy that sold me the car since I already kinda caught him putting his foot in his mouth on the price which he later found out that he did not know it was a 6 cyl. He admitted later that he was just blowing smoke up my ass, but since I came back so quick with the money he felt obligated to sell it to me. In the end this guy has sold me a car for close to $6k less than the car was worth and is now looking at around $4K worth of repairs. Something has to give because this situation is really brutal. Thanks.