I would basically say what you've said here. Something along the lines of:
"We are sad to report that [name] is back with us. When in a home environment, he started displaying behaviors that were not seen at the shelter. The adopters tried to work with him, but unfortunately it was beyond their ability, and they made the difficult decision to do what was best for [name] and bring him back to the shelter so that he could go to a home that was better equipped to care for him. While we're all disappointed that it didn't work out, we're grateful his adopters recognized that it wasn't the right fit and brought him back to us that the we can find a home where he can truly thrive. While we try very hard to ensure we're sending pets to the homes they need, sometimes it just doesn't work out, and that's okay! It gives the pet some time out of the stressful shelter environment and provides us with valuable information about their personality and behaviors in a home, allowing us to make a better match next time.
We now know that [name] needs a home that [whatever he needs, and end with some positives about his personality/behavior]
If you think [name] sounds like the perfect pup for you, please [whatever the next steps would be]"
Basically recognize that it's unfortunate that it didn't work out, express gratitude for the adopters for making a difficult decision in doing what was in the best interest of the dog, make sure to emphasize that it's okay if things don't work out, and then describe what you now know he needs in his home.
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Sam Maurice
Humane Society of Jefferson County
Jefferson WI
https://hsjc-wis.com------------------------------