They're not even abandoning the policy. The Tories have been clearly ramping up attacks on the £28b, and plan to make it a main thrust of their ad campaign for the election.
Labour saying they still plan to do everything, but only when the money is available. That could still be from early on, but it's harder to attack.
The Tories have always managed to convince a large part of the country that they know how to run an economy, when clearly they don't. Even after the absolute shit show of the last 14 years, Labour's lead in this area is miniscule, and having that £28b figure in the open, is obviously not helping.