I feel like this is NTA but I still feel like such an ass.

Me and my partner adopted a dog out of foster care like 4 days ago. When we were picking him up the foster lady mentioned "they thought he had a food allergy at the shelter so you have to get this kind of food but I think it might have just been shelter sickness".

Ok, cool! We bring the dog home and go shopping. The special food he needed was like.. 100 bucks for a 20 lb bag (He's a larger dog, lab mix. If he was tiny thus would last forever and I think it's be fine). Whoops! We decide to get a very small bag of normal food to try it out. The dog was very timid and both me and my partner were ehhh about him. Very sweet but not very.. companion-like. But shelter dogs take a while to settle so we waited.

A few days of eating the normal food and this dog vomits like 4 times in our apartment. Once in an unpacked bag, once on the carpet, twice (thankfully!) in the carpet less living room. It's AWFUL. So dog is definitely allergic and we were not interested in any kind of special needs dog that requires quite a bit more of a monthly cost (even compared to getting nicer grain free food over purina and stuff!)

We texted the foster lady back explaining we weren't capable of giving him the food he needs. I ran out and got the smallest bag possible of the expensive food for the weekend. She texted back saying it was 100% okay and that there was actually somebody else who wanted to take him home that had been disappointed he had been adopted!

So today we take him back but I feel awful. He's finally starting to warm up to us, wagging his tail when we come home, sleeping in our bedroom, following us around, etc. He had been in the shelter 100 days and had been found as a stray. I feel like I'm betraying this dog making him move around so much in a short period of time but the food allergy wasn't listed online or anything like that or we wouldn't have even bothered.

AITA? My mom found cheaper alternatives online of the food he needs but we had already texted this lady and I told my partner now he's going to somebody who already really wanted him. But I feel like a bad adopter to give him up after such a short period of time.

EDIT: We did indeed do the mix food thing when changing foods. I don't think that was the cause of his sickness - it is definitely allergies.

*EDIT 2: *

Thanks y'all. You gave me some good things to think about and I realize I'm just not emotionally ready for a pet yet. This proved it but also.. yeah. I lost 2 family dogs in the last year, both very suddenly, and I thought getting my own pet would help and.. I don't know. I just moved out for the first time and adjusting to adult life is difficult and I wanted something I had always growing up (a dog) and I jumped the gun. I wish I had figured it out beforehand but good news is that the dog (Billy) is on his way right now to somebody who was wanting to adopt him before we did and didn't get the chance (I was NOT throwing him back into a shelter or anything) so he is going to a loving home that will be able to take care of him.

Thanks for calling me out and helping me figure out how to be a more responsible pet owner. I actually feel better now about the whole thing because it helped me realize I do NOT need a pet right now, so ultimately sending him off was better for both of us. Y'all can be mean lol but it is actually awesome to see people so passionate about pets being happy - I know I sound like the worlds worst pet owner but I have always been really passionate about proper pet ownership and stuff. I just wasn't ready on my own.

Stay classy, AITA <3