apt-get install refuses to installs older versions of dependencies

[Fabian peter Hammerle <Fabianpeter.Hammerle@catalysts.cc>]

Dear Debian Users,

I set up an APT repository containing 2 packages: foo and bar

The repository contains 2 versions of foo: 1.0.0 and 2.0.0
and 2 versions of bar: 3.0.0 and 4.0.0

foo 1.0.0 depends on bar 3.0.0
foo 2.0.0 depends on bar 4.0.0

I can easily install the latest version of foo via
```
$ sudo apt-get install foo
[...]
The following NEW packages will be installed:
  foo bar
[...]
Setting up bar (4.0.0) ...
Setting up foo (2.0.0) ... ```

I get the same result when running `sudo apt-get install foo=2.0.0`

However, apt-get fails to install foo 1.0.0
```
$ sudo apt-get install foo=1.0.0
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 foo : Depends: bar (= 3.0.0) but 4.0.0 is to be installed
E: Unable to correct problems, you have held broken packages. ```

Currently, I workaround this issue by running `sudo apt-get install foo=1.0.0 bar=3.0.0`.

How can I instruct apt-get to automatically install an older version of bar as required by foo?

Or in other words: How can I make `apt-get install foo=1.0.0` install both foo 1.0.0 and bar 3.0.0 (dependency of foo 1.0.0).

Thanks,
Fabian