This is the law that complete forbids using any currency other than the national for internal commerce and disposes about how forex must work, which bitcoins do not comply.
http://www.planalto.gov.br/ccivil_03/LEIS/L4595.htm
This is a case where people were using dollars to pay for national contracts:
http://jus.com.br/revista/texto/8566/da-impossibilidade-de-p...