If a foreign entity were to buy slots and lease them to United, they would consider them an investment which would have to return a profit just as any other investment that a company would make. Any banker or investment company in the US could buy US’ slots and lease them to airlines if they wanted to since slots theoretically are pretty valuable assets.
However, worldwide nearly all slots are held by airlines and not be investment companies. Airlines sell and lease slots to each other but not many others get into the process for a couple reasons. First, slots are extraordinarily perishable – they are nothing more than a right to operate one flight per period (usually per day) at a specific time. Second, they are highly specific, many slots, including US’ slots at DCA, have restrictions attached to their use – within 1000 miles by aircraft of a certain size. They are very dependent on an airline’s schedule and essentially only the network planning dept. of an airline knows where slots would be needed; an outside investor would take a large risk by buying an asset for which someone else could ruin the value. And finally, and perhaps most importantly, it is highly likely that the US government will change the rules for slot holdings at LGA and DCA – the only truly constrained airports in the US from a slot perspective. Given that US is a major slot holder at DCA and LGA, there is considerable opportunity for the gov’t to change slot rules to allow LCCs greater access to LGA and DCA and then proclaim that airports in the US are open for business to all new carriers. They cannot now say that and many speculate that part of the reason they refuse to let go of their influence on the airline industry is because it has not yet been a market where there is truly free access. Incidentally, the US is by far the most liberal country in the world in allowing new entrant airlines access to its airports so being able to prove would go a long ways to denouncing those who say that the US government is protectionistic of its airlines.
And even if the government doesn’t intervene, legacy airlines and LCCs alike can come up with all the cash that is needed to acquire the slots and facilities necessary to operate at both LGA and DCA. The only way those slots are not likely to come on the open market is if US creates a business plan which involves investing in a totally refashioned airline, including aircraft, hubs, and slots and gains an investor to back them by creating a business plan that is so uniquely US that no one else would be interested in investing in the business apart from what it could be as a knock off off US.