RAW speaking, it wouldn't work to begin with
In the description for Revivify, it states that the material component is:
(diamonds worth 300 gp, which the spell consumes)
So you have two problems; it's stating diamonds as a plural meaning that it is expecting more than a single diamond, and it is giving an exact amount.
If you compare this with say, Raise Dead, it has a very different description:
(a diamond worth at least 500 gp, which the spell consumes)
In this case, it calls out that it must be a single diamond, but that one diamond can be worth 500 gp, or more. That "at least" wording is missing from Revivify.
That being said, I'm sure most DM's would allow some leeway against RAW, but not consume.
As a DM, I'd say that for Revivify, if you happened to have a single diamond that was worth at least 300 gp, I would allow it. But it would always consume the whole diamond. Magic doesn't give money back...
As an example, the Find Familiar spell asks for "10 gp worth of charcoal, incense, and herbs that must be consumed by fire in a brass brazier." If you put in 15 gp worth of charcoal, incense, and herbs, would you expect the extra 5 gp to not burn?