Which is correct – "whoever" or "whomever" in this example? [duplicate]

I say that "whoever" is the correct grammatical choice in the following sentence:

Give the raise to the hardest-working employee, whoever/whomever that may be.

I say "whoever" is correct. The reason is that, when we isolate the "whoever/whomever" clause ("whomever that may be"), the phrase converts to "It may be he". "It may be he who is the hardest-working employee", not "It may be him who is the hardest-working employee". Thus, I believe "whoever" is the correct choice here. Am I right? And is my explanation in determining "whoever" on point?

This question is different because the "whoever" clause is at the end of the sentence.

According to the sentence mentioned in the question, if there is only one hardest working employee who has to get the raise, using whoever is correct.

Give the raise to the hardest-working employee, whoever that may be.

Reference Link