How can gold be traded to other players Diablo 3?

So now that the Auction House(s) are down, how can you legitimately trade gold to other players?

For example my brother wants the recipe for the new infernal machines and it costs 5,000,000 gold. I have 127m that I have massed up from before the AH was removed but he only has 1.2m.

How can I help him get the book since it's account-bound on purchase and I am seemingly no longer able to drop gold or trade it to him (even though the gold option is still in the "trade" menu).

You can't trade gold. It is no longer possible to transfer gold between accounts in any way or form.

From patch 2.0.1 notes:

Gold can no longer be traded

It may be possible to purchase items from the merchants and giving it to him to sell, but this will be at a massive loss rate.

If you wanted to help him along, there are still several ways to assist in his gold income:

• Take him along while you do bounties. Each completed bounty earns a fixed amount of gold (based on difficulty level).
• Take him along for rifts. The rift boss usually drops ~ 50 blood shards, which can be used to purchase 10 pieces of armor at the gambler, each selling for somewhere around 3-5k a pop. This can add up quite fast. You also earn some gold completing the rift quest.
• Give him all the rares you find as you play. The rare sell values has really gone up in RoS and making a few million doesn't take long at all.