Você tem que instalá-lo a partir do stretch, adicione as fontes do debian 8:
echo "deb http://mirror.hetzner.de/debian/packages stretch main contrib non-free
deb http://http.debian.net/debian stretch main contrib non-free
deb http://http.debian.net/debian/ stretch-updates main contrib non-free
deb http://security.debian.org/ stretch/updates main contrib non-free
" > /etc/apt/sources.list.d/stretch.list
e fixe-os abaixo de todas as fontes jessie com
echo "Package: *
Pin: release n=jessie-updates
Pin-Priority: 500
Package: *
Pin: release n=jessie
Pin-Priority: 500
Package: *
Pin: release n=stretch-updates
Pin-Priority: 499
Package: *
Pin: release n=stretch
Pin-Priority: 499
" > /etc/apt/preferences.d/stretch-manual-only
Agora você pode instalá-lo com
apt-get update
apt-get install dfc