smc

When Mr.A sells the house at a $10\%$ loss, he sells it for $9000(1-.1)=8100$. When Mr.B sells the house back to Mr. A at a $10\%$ gain he sells it for $8100(1+.1)=8910$. Therefore Mr. A has lost $8100-8910=810$ dollars, so the answer is $\boxed{\text{Mr. A loses }\text{textdollar}810}$.