Exploitable uses
I'm assuming the intended use of this spell is to sacrifice your own HP to save someone else, and so any use that doesn't result in the caster being depleted of HP is abusive.
Recovering the spent HP
The basic abuse of this spell is to grant someone a temporary hit point (THP) buffer, then use a healing spell or other source of healing to recover the hit points spent. The advantage of this trick over using the healing spell normally is that you can have the extra hit points in advance, preventing instant death from massive damage, lost turns due to unconsciousness, or the problem of the healer having to actually get to you in combat.
It also works with forms of healing that are too slow to be practical in combat, especially short rests and the (already ridiculous) healing spirit spell. The Blood Transfer guy can transfer a large number of HP to someone, splash around in the healing spirit fountain, and (if he's quick) do it again, continue doing this as many times as they want to spend spell slots, then do one more big transfer, take a short rest, and burn hit dice to recover.
Similarly, a druid with Wild Shape can give you the HP of a body he's not even going to keep.
All of these tricks require spending resources to get the HP back--it ain't free. The problem is that all of those resources come back eventually if you survive, and the THP buffer makes it easier to survive anything. If you can convert all your spell slots and hit dice and Wild Shape uses and everything into a giant THP pool, then the solution to most problems becomes "do that, then just beat the monster to death with your face." This is boring, and you shouldn't make the boring tactic the most practical one.
The only way to really fix this is for Blood Transfer to reduce the caster's maximum hit points, like a vampire bite (which is literally a blood transfer, so there's precedent). That sounds harsh, but combined with a reasonably short duration (like "until your next short rest") it really should only block abuses of the spell.
Laundering THP
There are spells like armor of Agathys and false life that give THP with restrictions (self only, short duration). Blood Transfer does not share those restrictions, so it can be used to convert "5 THP to yourself for an hour" to "5 (or more) THP to anyone until their next long rest".
The simple fix for this is to require the spent HP to be real hit points, not THP.
Not quite exploits but discuss with the player anyway
Duration
Blood Transfer doesn't say how long the THP last. By default, this should mean they last until the next long rest (see the last sentence here). It might be smart to say this specifically so the player doesn't think they last forever. (Thanks to Medix2 for pointing this out.)
Disciple of Life and Blessed Healer
Life-domain clerics get various improvements to spells that restore hit points. Granting THP really isn't restoring hit points, so this shouldn't be an issue, but make sure the player knows that before they try to set up some infinite combo around it. (Thanks to RallozarX for pointing this out.)
THP won't actually save you from death.
If you have 0 hit points, receiving temporary hit points doesn't restore you to consciousness or stabilize you.
This is a problem for the intended use of the spell. If this spell doesn't stabilize, I'd say that in advance, or else it's going to be a nasty surprise the first time they try to use it on a dying guy.
(If you fix it so that it does stabilize then it's a straight upgrade of spare the dying. I don't think that's a bad thing--someone might actually use it. But it also significantly changes the aesthetic of the spell, so be cautious.)
This isn't damage.
"Sacrifice hit points" is not language the game normally uses, but you should make it clear that the hit point loss is not damage. It can't be reduced by anything that reduces damage, it doesn't trigger anything that happens when you take damage, you can't be resistant to it, etc. For further protection against shenanigans, I'd specify that the spell only grants THP equal to the actual loss in hit points, not the number you would have lost in some hypothetical case.
