I'm sorry to hear this happened.
Just wanted to say I was in a similar situation. I worked with the local animal shelter (the city's "SPCA") and there was a Rottie mix who was very fearful, but once you earned her trust and showed her what was expected of her, she was one of the most wonderful dogs - very pretty at that. She was noted to have potential dog-dog aggression due to this fear and was put into the adoption program to give her a chance because the shelter staff fell in love with her after also earning her trust.
Her name was Sasha and after working with her for about a week, she was adopted by a family who lived out in the suburbs in a neighborhood that allowed dogs to run around off-leash.
2 days after Sasha's adoption, an email was sent out to the volunteers who were involved with Sasha's care when she was in the shelter. We all found out Sasha was put down for dog-dog & human aggression per
shelter policy. Sasha's new owners didn't want to lose their new dog like they did their previous (ran away), so they kept her on a leash. Wanting to socialize her in hopes of getting her acquainted with everyone in the neighborhood, they took Sasha out to meet other dogs who were OFF-leash and roaming around.
(The adoption process at the shelter is quite lax - come in, fill out an application, pick a dog; leave with dog. Dog barely knows their new owner.) Naturally, Sasha felt threatened by all the new dogs and people. From what I heard, she was surrounded by dogs and people. Had the owner noticed she had NO escape route when that was what she needed most before she was forced to resort to the "fight" response, Sasha would still be alive. Sasha attacked an off-leash dog out of pure fear, rendering mild damage. The owners were shocked and reprimanded her on the spot; she turned on her owners too.
She was immediately returned to the shelter; and because regardless of the situation that caused her aggression, she did in fact bite both a human and another dog. The owner of the injured dog intended to pursue charges against Sasha owner if Sasha was not put down.
Everyone knew she was wrongly put to death; but policy could not be changed for one dog. I just wish she could have been an exception to the rule. The staff literally had NO time to find a rescue group to save her. I'm not sure about this, but I think another, more suitable dog was adopted out to the family because Sasha "didn't work out".

If it were up to me, I wouldn't allow it.
Everyone at the shelter absolutely loved Sasha. I personally loved Sasha because she was a mini-Rottie. Perfect size for me. I think she was a rottie/lab mix because her head had Rottie features to it, but more the look of a lab's block head. Haha. I miss her.