On the credibility scale, "We owe you one," is right up there with, "The check is in the mail." The fact is that you are dealing with an unbelievable manager. As other choice assignments go to some of his other pets, don't be surprised to hear, "We owe you two or three or four."
If he were truly committed to keeping his commitments, he would have said more to you than some vague throwaway line. For example, if he had met with you, discussed the decision, and pointed out the kinds of specific opportunities that he will try to steer your way in the future, that would be a far better sign.
Since he did not discuss these opportunities with you, then you should discuss them with him. Meet with him and ask him to elaborate on just what it is that he owes you. Be sure to get his inputs regarding any actions that you should be taking in order to be fully qualified when such opportunities arise. As time passes, let him know about your successes.
Unfortunately, you are dealing with a manager who plays favorites, does not keep his word, and communicates marginally. You owe yourself something better than this.
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