Was Bisher a Moslem?

We learn from the incident of herdabia that it is allowed to hire out a pagan against fellow mushrikin. As long as you do not tradict the letter of the treaty, then this is something that is permissible. Where do we learn this from women? Women migrating because the treaty said, no man shall come. But in this case, the sahaba, the process, they stuck by the letter, even if it meant the spirit was lost. And also, we have another evidence here, and that is that, a the processon, was told that you're not going to allow somebody to live in your midst. So he actually allowed him the opportunity to go.