Maybe he hopes that Calculus regrets it at the moment he dies. It's sort of that you'll-pay-for-this kind of thinking, when someone regrets upon dying...
"A pity he didn't agree to work for us, but who knows, he may change his mind some day." This could be read as 'One day soon Calculus will regret turning us down.'
Excellent point! I was confused as well when re-reading the story knowing that Calculus did not actually have too much time to regret not working for Sponsz when he was in Borduria but indeed the best explanation is probably the assumption that Sponsz is merely giving the readers a sardonic hint that Calculus’s days were numbered (or at least that’s what he thinks!) and that the dying Calculus’s last thoughts might have been something like “Great sunspots, if I had given my plans to those nice gentlemen in Borduria, maybe I wouldn’t be bleeding to death right now because in return they got me blown to pieces inside this jeep with my friends”.
Or perhaps some lines are not to be taken too literally. After all, later in “Tintin and the Picaros”, Tintin and Haddock say to Sponsz “We meet again, Colonel Sponsz”, when the line should have been “We meet AT LAST, Colonel Sponsz” as Tintin and Haddock never actually met Sponsz in Borduria. They did watch him passing by at a distance but they never had the pleasure of being introduced.