I sent out a book that the receiver mentioned they probably would not get, as they had a postal mixup at their end, or had moved or something.

I received the book back, but the postmaster won't release it to me until I pay an additional $2.38.  They said that media mail doesn't go back "free".

Does that mean that I have to pay $2.38 to get it into my hands again, and an ADDITIONAL $2.38 if I want to remail it to the same address again?

Just trying to decide whether or not to let them keep the book  LOL as it won't be worth my having to spend $7.00 to swap a book not even counting DC

That is correct.  Only Express Mail, Priority mail, and First Class mail have free forwarding and return.  Media Mail (a subclass of Package Services) is forwarded and/or returned, Postage Due. 

If the requestor is the one who screwed up on the address, then she owes you a credit for sending this book. This is from the Terms of Use:

Address
Members are required to provide a current, dependable, and secure mailing address at which items may be reliably received. In the event of a change of address, Members are responsible for updating such changes to their Site account. Failure to report change of address in a timely fashion may result in loss of Book Credit for misaddressed items.

If the requestor won't honor this, then you can contact the site admins by uisng the Contact Us link under Company at the bottom of the page.

What Cathy said

thanks, everyone

If the problem was the post masters fault they will refund you the extra $2.38. I had a simular problem in Auguest also.  I ship 2 books to one lady and the postage the post master put on came off during delievery. I had to pay the extra $2.38, but I did go in and made a companient and they reship the book at their cost and refund me the $2.38. But the lady I ship it to never gave me credit for the books and won't return my pms to her.

Christine