If the candidate still has money in the bank, they can more or less do whatever they want as long as it doesn't go directly in their pockets. Typically early in a presidential race, a campaign will start to run out of money if they're not that viable and can't get more donations. If they are viable and can go late into an elections cycle, (like Bernie and Hillary last go round), then one candidate (in this case Bernie) had a good chunk of cash in the bank. You'd need to dig into his FEC filings, but I believe he more or less sat on what he had left until this cycle. Now if the candidate plans to not run again, they can donate it to another campaign, donate it to charity, donate it to a PAC, use it to campaign for someone else, etc.