How does a person become a billionaire? Either by inheritance, or by founding or investing in a corporation that achieves market capitalization in excess of $1B times his equity stake.
Speaking fees, investing his salary on mutual funds, writing books, etc. will not work. At 100,000 USD per speech, you would have to give 10,000 speeches that would take 192 years at a rate of one speech per week. At $2.5M per book, you would have to write 400 books. Would the 400th book by "the most interesting man in the history of mankind who also was the president of the USA" really sell?
His sole chance is to invest in start-up companies and then use all the powers of his office to make sure that those companies are hugely successful. Whether this is legal and ethical under your requirements 2 and 3 is highly questionable. 207
Before 2017, I would have said this is clearly illegal but ethics rules regarding presidential business interests seem more murky under the current administration, so you may have a way out. Perhaps a family member can invest on his behalf. Think of it this way, if it was easy, every president would have done this. Who doesn't want to be a billionaire?
So, what start-ups is he going to invest in? Things that almost every other investor sees as non-viable but a president could potentially make viable.
* Human germline genetic manipulation, the designer baby company.
* Reckless gene therapy for every illness. The 21st century cures company.
* The cognitive enhancement company. Free, legal stimulants for everyone (competition from mainstream big pharma is an issue).
* The Mars colonization company.
* The green energy company
* The bog peat burning company
* The armed drones for civilian home defense and big game hunting company (competition from current defense contractors would be an issue but you could pass a law to the effect the same corporation cannot produce both military and civilian drones)
* The US affiliate of the one and only North Korean company and bank allowed to do business in the US
* The one and only US company allowed to export dual-use technology to Russia
Basically anything that he can strong-arm the NIH, State department, EPA, Department of Energy, NASA, etc to make unexpectedly profitable. 207