Let's say it was 400k in gold bars. He'd have a much easier time demonstrating purchase of that gold over time through receipts at least kept by the commodities or gold exchange the metals were purchased through. But let's take it as assumed that this is indeed what he says it is: simply a life's worth of scrimping and saving. That'd be bundles of 1's, 5's 10's and 20's in that safe. That's going to perk the eyes of any police officer, especially when drugs are later found.
There's still that lingering question - why did two guys from Tennessee come driving into Ohio to rob a house that just happened to have marijuana and 400k in cash in it? Bet the statement from the robber who escaped after stabbing the man's son has a lot of play in here. I'd imagine he's been singing like a bird in order to avoid murder charges.
I have a feeling junior has a lot more to do with the money than dear old dad. If dad never had a bank account, that backs up his story. Of course, he’d have to show how he converted his paychecks to cash.
Coincidence, the man or a family member told the wrong person about the money in his safe, etc.
Let the government use the thieves' statements to prove the confiscated money was a result of criminal activity, instead of forcing the man to prove a negative.