"I was about to offer myself, sir," said the priest; "it is
our mission to forestall our duties."

We had this word perhaps six months ago, meaning a crime consisting of buying up all of some necessary commodity in order to raise the price.
I don't know what the French word was, but it seems clear to me that the translator made a poor choice.
Incidentally, Dumas is getting nuttier and nuttier. In chapter 101 he has Monte Christo giving a girl a medication which will convince two experienced physicians that she is dead, but capable or staying pulseless and breathless for three days, and then making a complete recovery.
I'm beginning to think Dumas wrote this while high on the hashish which he praises so extravagantly.