Why is your brother-in-law willing to pay six hundred dollars for an untrained, ill mannered, destructive, escape artist?

Sure go ahead and sell her to him. After you've had her spayed. At least she'd have the experience of socialization with another dog before she's dumped again.

From what you've indicated it's not likely that there is an ethical breeder involved that would be willing to take her back and rehome her. If you care anything for the dog at all, contact a Boxer rescue and give her the chance she deserves to find a forever home where she will be loved and appreciated.

If given the opportunity; at only eight months of age she should be extremely trainable and will no doubt make someone a wonderful pet.

The sooner she is removed from her current situation--the better her chances will be!