A pension bet is when two (or possibly more) djinn wager three wishes on a single event. In certain causes, a double bet can be formed where six wishes are owed. To not perform the conditions of a pension bet can be considered a great insult and could even result in a djinn duel.
Rabbi Joshua, a famous djinn mathmatician was known for making penison bets. In the seventh book he is shown to have made penison bets against both good and evin djinn alike. Nimrod challenges him to a penison bet in exchange for looking at the Rock Archives. Nimrod wins the bet against Rabbi Joshua and he is forced to comply.