The £1bn is earmarked in the Labour manifesto for extra hospital and dental appointments and school breakfast clubs.
The issue is that the concessions made when Jeremy Hunt unexpectedly scrapped the non-dom scheme, were designed to reduce the incentive for wealthy foreigners with a permanent home abroad to emigrate.
About half the money raised from the wider abolition plan was already forecast to be lost in changes of behaviour.
The revenue raised was assessed by the OBR in March to be highly uncertain.
Small changes to assumptions about emigration, for example, could mean the planned additional tightening of the plan could raise very little.
While no final decisions have been made, some watering down or phasing in of the decision to apply inheritance tax to trusts, and a discount on bringing in foreign income next year, is being considered.
The Treasury is adamant that any further changes to the regime should be shown to raise money, and that non-dom status in general will still be scrapped.
