This works as you suggest, sort of
Nothing prevents an object from being used as a component for multiple spells.
For magic jar, you need...
a gem ... worth at least 500 gp
and for instant summons you need...
a sapphire worth 1,000 gp
A sapphire is a gem and 1,000 gp is at least 500 gp so the component qualifies for both spells.
Breaking the Sapphire
Since both spells were cast using the sapphire, we need to make sure they don't get in the way of each other. From the Spellcasting rules:
The effects of different spells add together while the durations of those spells overlap.
This means that the sapphire is influenced by both spells. Which means...
If the container is destroyed or the spell ends, your soul immediately returns to your body.
At any time thereafter, you can use your action to speak the item's name and crush the sapphire.
Who has the dagger?
There is one problem. The dagger will be summoned in the possessed friend's hand.
When you crush the dagger you are currently occupying the body of the target's best friend. When the spell says...
The item instantly appears in your hand
..."your hand" refers to the host body you occupy's hand. So while the dagger does teleport into the vicinity at the same time you return to your original catatonic body, you will have to retrieve the dagger from the friend's hand before being able to strike at your target
Keep in mind that the possessed friend has been aware of its surroundings this whole time and may be knowledgeable about the workings of instant summons (which you cast after magic jar).
the possessed creature's soul can perceive from the container using its own senses
He/she may have figured out your plan during his/her captivity and be ready to stab at you after he/she regains his/her body. You can probably rely on surprise and a high Initiative to have a turn to get the dagger while the target is surprised and strike before he/she can react. However, the friend of the target might not be surprised. Remember that surprise is in the hands of the GM:
The DM determines who might be surprised