Bribing players to help you can be
- Treasures earned from winning the combat;
- Items you have in play that you can trade after the combat;
- a promise to play a card you could legitimately play on another player that would be helpful to that player (e.g., a GUAL, Wishing Ring);
- a promise to not play a card you could legitimately play on another player that would be detrimental to that player (e.g., a Curse).
The first two would definitely have to happen after the combat, since you can't trade during combat. The other two, however, could happen at any time. What you can't do is promise to give someone an Item card (or any other card you couldn't actually play on another player, but see above) from your hand unless you can put it into play after the combat.