I have a player who is leaving the game, his own reasons. His character (A) "rescued" another character (B) during an encounter where character B was brought into the game. Character B is cursed, forced into servitude and protection of another. So, B is bonded to A because of this curse. Curse was put on the character B by a powerful Fey being. For the backstory of B, we stated that the "bond" was passed to another when the current "master" was killed and it went to the closest entity to B.

Not wanting to kill character A, just cause, and rather saying he went home to confront his father, Demon who thinks is son is nothing but a coward. The party is in a city, just another day or two before they continue on, no encounters were planned for the rest of their stay.....not sure what else to give you all so if you need anything more just ask.

Looking for ideas on how this "bond" might pass to another entity if I dont kill player A. Player A will not be present so how do I RP this out?

Quote from PeachkaSlayer >>

Looking for ideas on how this "bond" might pass to another entity if I dont kill player A. Player A will not be present so how do I RP this out?

The easiest option I can think of is that the entity B was originally bonded to isn't really dead or got resurrected for Plot Reasons, so the curse never actually passed on (or reverts back)

Maybe the curse has some story-book sounding condition for removal. Something weird and specific as is befitting the fey, like brushing your teeth with horse hair, or sleeping on the roof in the middle of a Tuesday. Maybe the player can beat the fey King's champion at some nonsense contest in order to release them from the curse and allow character A to retire without character B bound to them. 

One time in a fey themed dungeon, I ran for a party that challenged a hag to a game of charades and I actually had the players play it irl. The hag cheated using Minor Image (technically she didn't cause the only rule she stated to the players was that you couldn't speak and Minor Image doesn't have a verbal component), which the players were really annoyed/amused by.  Just an example of strange challenges that fit the fey mindset. 

my 2cp worth.

If player "A" is going away you could have their character just change shape into some form of trinket such as a ring or necklace. Think of it like the talisman some Warlocks can get when they take the Pact of the Talisman. In this instance though It serves no real purpose for Player "B"'s character other than being something they feel the need to look after/protect/refuse to sell and then, should Player "A" ever have a change of heart and want to come back to the game, you can just have the talisman reform into player "A"'s character.

If there is some really good RP form Player "B" around it then you might given them some small boon such as a d6 inspiration once per long rest that they can only use if they have the trinket on them.

Random thought.  You mention A is going to confront his father, a Demon.  What if a rival fiend had the idea to nudge this.  A powerful enough Fiend should be able to either break the curse, or transfer it, to, perhaps, as suggested a ring, or bracelet or something.  The rival would do this to prompt the encounter/faceoff, in hopes of weakening or, depending on what the player's narrative wanted to be, eliminate the father.

Maybe B wanders into the middle of a spellstorm or something and the wild magic distorts or lifts the curse?

A God takes interest in A and deems he should have a protector and transfers the curse from B to a wolf, or dog, or some other form of "protector" for A.

Keep in mind you, as DM are the Ultimate God of this world and situation.  Any idea you come up with that you think is cool and will give some satisfaction to the players, as well as resolve the issue, is fair game.  Your world, your rules, your events....you know your players, so something they will think is cool will fly pretty easily, even if it stretches plausibility a little.