Some papal elections during the rennaisance were notorious for allegations of bribery. For example the 1492 conclave which elected Rodrigo Borgia as Pope Alexander VI. The amount needed for bribes were considerable, e.g. Wikipedia mentioned "four mule-loads of silver". Also, elections were not frequent enough for people to simply trust each other by habit. How did people ensure that such a risky transaction was honored?
If the payment was first, how could the payer know that, after that big payment, the cardinal would vote as requested? If the payment was later, how could the cardinal know that he would be paid?