Sometimes an installation fails due to inappropriate dependencies. This is usually due to additional repos. This can be remedied by downgrading the not matching packet. As an example here the output of apt-get install openssl-dev:
libssl-dev : Depends: libssl1.0.0 (= 1.0.1t-1+deb8u5) but 1.0.2j-1~bpo8+1 is to be installed
Recommends: libssl-doc but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
A downgrade of libssl is easy:
apt-get install libssl1.0.0=1.0.1t-1+deb8u5
Reading package lists… Done
Building dependency tree
Reading state information… Done
The following packages will be DOWNGRADED:
libssl1.0.0