Stanley's reaction to the loss of Belle Reve is that he is furious and is curious to what happened because he claims Belle Reve as his own as well.

In actuality.... Stanley resents the loss of Belle Reve on Stella's behalf. He doesn't trust Blanche and feels as if she is swindling her sister. As a strong male figure and Stella's husband, he feels he has a right to question the sale... he also sees Stella's money as his own.