[SalesForce] Future Method calling Another Future Method

Definitely you can use an HTTP callout as an escape mechanism. But this is pretty naughty. You'll need a Remote Site Setting and a Session ID and a global class etc etc etc (red flags, alarm bells).

May I humbly recommend the use of System.Queueable call instead of @Future. You can create a class that is constructed with any kind of argument that you want to hand in, like so:

public class DeferredHandler implements System.Queueable
{

    private Map<String,String> key2val;

    public DeferredHandler(Map<String,String> key2val) {
        this.key2val = key2val;
    }

    public void execute(System.QueueableContext objContext)
    {
        //do your @Future stuff here with this.key2val
    }

}

And invoke him from inside your batch like this:

//defer stuff
System.Queueable job = new DeferredHandler(key2val);
System.enqueueJob(job);

There is some literature about this in the docs and Josh Kaplan's awesome Queueable blog post.

The list of primitives doesn't include Map, Set, or List. Therefore, what you have is a collection of non-primitives. They've also explicitly excluded Object, so you can't weasel your way around that limitation that way, either. You cannot include any nested collections at all, including things like List<List<String>> or Set<List<String>>, or any other combination of nested collections.

This restriction has to do with how future methods serialize their parameters; it literally cannot support complex data types. It was originally placed on future methods to limit the complexity of serialization, which is why salesforce allowed 50 future calls as opposed to five Batchable calls per transaction.

Since then, we've been given Queueable, which suggests that future should enjoy the same serializing features, but it hasn't been upgraded, and probably won't be for the foreseeable future, because we have a newer feature that gives this this ability already. If you need future-like behavior with complex types, use Queueable instead.