You are forgetting that despite being an artist of a talent, Claude is still a businessman as well. If Yune really wanted to sell the kimono, like she insisted against his initial word of caution, then who was he to deny it? He only began to regret it when he learned it was a memento, which was well explainable by how he was attached to the light fixture created by his own dad. So, it was only at that point he knew Yune wanted to sell the kimono not as a simple financial compensation and to aid the store, but out of insecurity and fear of being rejected and left alone in a foreign country.

But yeah, he probably shouldn't have been so eager, especially when he was dealing with a kid.