AITA for buying my nephew a toy his parents couldn't afford?

So, my nephew (11M) is a huge Transformers fan and has been collecting Transformers toys since he was eight years old. He really wanted this particular toy, Devastator, which was a giant combiner transformer from Transformers 2. The only problem was that he wanted a specific version of the toy that was made years ago and was no longer widely available. When I looked online, I found that the toy was over $400, and his parents refused to buy it for him because of its high cost.

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Feeling sorry for my nephew, I decided to try and find the toy for a cheaper price. I searched online but couldn't find it cheaper anywhere. Eventually, I made a post on Facebook and found a seller who was willing to sell the toy for $230, which was a lot cheaper than the online price I had seen earlier. I decided to buy the toy for my nephew and gave it to him as a gift.

However, a few days later, my sister called me and started yelling at me for buying the toy for my nephew without her permission. She was furious because she had specifically told her son that he couldn't have the toy, and she felt that my actions made her look bad and poor.

Also, she did not want her son to have the toy because he was doing poorly in school, and she wanted him to focus on his studies instead of playing with toys. She told me that I should have consulted with her first and that I should not have just bought him anything he asked for.

Am I the asshole here?