Am I in the wrong here? :(

I'd say don't look a gift horse in the mouth. As annoying as this is (and the sort of thing my mother would do), you now have a DJ that you couldn't afford, despite not being bothered with having one, but as I said on another post similar, it's like having a sandwich without a filling. In my opinion a party is not a party without a good DJ playing to the dance floor. Or it just becomes a gathering with music in the background. If you just wanted a gathering with background music and no dancing, then just take control and cancel it and tell her its not that kind of party.

You could use this to your advantage. Find out from her what she believes should be included at this party and tell her your not booking it and let her get carried away! before you know it you'll have a completely free party ?

It's understand able to be annoyed that she went behind your back. But I think it's nice that she did it for you - booking AND paying. She probably thought she was helping - Did you tell her it was because of cost only? She is probably overbearing because she is excited. Accept her gift (but may be keep your plans closer to your chest next time if it bothers you so much.)

To me, it sounds like she was trying to do something nice, especially if she thought it was just financial reasons that you were choosing not to have one. However, I can definitely understand why you would be annoyed! Especially if she then tries to go against decisions that you make for the actual wedding day...

If it were me, I would sit down with her and my partner and just discuss it to make sure she doesn't do anything again without asking, even though you're appreciative of her splashing out to try and help!

WOW! Personally, I would be extremely annoyed if this were my engagement party and my MIL did that. For one thing, I am SUPER FUSSY about the music that gets played, and would prefer my own choices, or to have my choice of DJ. But the more important thing - if she's like this now for your engagement party, what will she be like for your wedding?!

I think if you don't nip her interfering in the bud now, she will cause you a lot of problems in your wedding planning. Just letting her get away with such an interference will set the bar for her being able to do it more as the wedding plans are underway. So I would find a way to address her about it.

If you decide to accept the DJ that she has booked, I think you should do it by telling her it is only on condition that she doesn't pull a stunt like this for something at your wedding.

What matters isn't that she has saved you some money - but that she specifically went against your wishes, behind your back. It could have been food, decor, the wedding car, anything...but she wasn't respectful about your choice. She sounds very overbearing, and it's not her wedding!