Active to passive voice conversion for "He has money."

If you change the verb then you won't have the passive voice of that specific sentence. In fact, "is" is neither active nor passive; it is a true linking verb. You can freely switch the sides without changing the verb (although the result here sounds a little antiquated):

• My apple is with him.
• With him is my apple.

There's a formula (so to speak) to switching from active to passive. It can be used on any active sentence, but the resulting passive voice sentence is not always idiomatic (and it's sometimes so bad as to be unintelligible).

The sentence "money is had by him" is the passive voice equivalent of the sentence, but it is not idiomatic. In this case, it's this particular use of the verb that causes the problem but there are other reasons why the passive might not work.

However, I will note that there are other uses of "have" that are idiomatic in the passive, such as the cliché "a good time was had by all".

For more information on this, see the book On Voice in the English Verb.

(Copied mostly from my answer at ELL.)