After years of lobbying from technology companies, the White House this week set the ball rolling for a tax holiday that would slash the cost of bringing home the mountain of cash that Silicon Valley has stranded in offshore accounts — though with scant detail on how it might work.

President Donald Trump wants to twist the arms of US companies to make them invest more in growth at home, and there are no companies with greater financial wherewithal to invest than the tech giants. But if jobs are the objective, tapping foreign cash reserves is unlikely to produce the prize the White House is hoping for.

For a start, the headline numbers overstate the amount of stranded cash that could be put to use. Tech groups are sitting on about half of the $1.3tn that US companies have lodged outside the country, according to Moody’s Investors Service. But much of that money has already, in effect, come home through the back door.

Consider the five tech companies with the biggest foreign cash positions: Apple, Microsoft, Cisco, Oracle and Alphabet. Together, these companies have added $314bn to their offshore cash holdings in the past five years, bringing the total to $512bn.

Over the same period they have also borrowed $218bn and then used the money in ways they could not have done with the offshore cash without paying tax. Put another way, this means they have found a release valve for 70 per cent of the foreign cash they generated in the period.

It is not hard to find where the money went. The top five companies have paid out $350bn through stock repurchases and dividends in that period. So while shareholders cannot get their hands directly on foreign profits, that has not stopped a massive giveaway to investors.

The drawback of this arrangement is that the money left sitting in overseas accounts continues to pile up at a steady rate and it earns a negligible return, while the matching debt has a cost. But at least the tech giants have been enjoying some US tax relief on their interest payments while they wait for a solution to this mismatch.

The backdoor repatriation is just one of the factors that makes the tech offshore cash mountain less of a prize than it seems. A large slice of the cash has already been handed out, leaving a mountain of debt to be paid down.

Another complication is that it is hard to see how the White House can use a tax holiday to nudge the big tech companies to ramp up employment in the US. Their hiring and investment has not been held back by a lack of liquidity. The debt financing has supplemented a powerful onshore cash flow that has supported an expansionist period for the largest tech companies, as they try to stake out territory in big new markets such as cloud computing.

The five biggest holders of foreign cash spent $45bn on research and development last year, so they — unlike other large US companies — cannot be accused of under-investing in their businesses.

To force investment, the White House could attach conditions to any cash that is returned under a temporary tax holiday. But that might make companies reluctant to bring their money back. During the last tax holiday for foreign cash, in 2004, less than 10 per cent of the money held offshore was repatriated, according to Moody’s. Even then, almost all the money that came back found its way into shareholders’ pockets through buybacks or other means, rather than being invested in expansion.

There are other levers that politicians could use to try to force tech companies to plough more into their US businesses. Republicans in Congress are proposing a border tax that would put a levy on imports. That might tip the scales for a company such as Apple and encourage it to invest in more US manufacturing.

But the White House tax plan announced on Wednesday did not mention a border tax. Tough talk about the end of globalisation and a trade war might spur Apple and others to invest in local production as a hedge. But it would take years to build an electronics supply chain in the US to match that in China, making this a poor response to short-term political risk.

The coming White House photo op is easy to imagine. Tech executives will stand beside Mr Trump and talk about how releasing all that offshore money can bolster US growth. But it will be shareholders rather than workers who will be celebrating the most.