From what I understand of the situation you've described, the Predictor doesn't seem to factor you opening Box B into account at all. In addition to that, you've not mentioned the Predictor's accuracy rate.

As such, it seems that the Predictor is relying on you checking Box B and finding it empty - because if you do, then you're definitely going to take the thousand euros and leave. If he put a million euros in Box B, then there's nothing to stop you taking both, and his prediction would be wrong.

I think the Predictor is a bit of a prick!