I suspect there are few real-world examples where "you could just say X" actually works... but in this case the story is that a random "weed dealer" killed not a random person, but a specific person known to the "weed purchaser"?

Who would believe that story?

How does prosecution prove the payment was connected to the hitman and not the weed?

Seems like they'd need to seize the hitman's digital communications, at which point the jig is probably already up.

It's "beyond a reasonable doubt," not "beyond all doubt."

Evidence.

A grand jury must look through the evidence and conclude that there is enough there to possibly result in a conviction.

Being actually innocent isn't enough to protect you from the FBI, so no, any suspicious trail at all is too much. Once you're a person of interest, any of the many possible opsec failures is enough to build a case. And for (attempted) murder, the spouse is usually a person of interest.

Yes you can do it with any money. What's your point?