Trump’s intention may be clear, but the language of the order is not clear and that is what may rule the day.
If the language of the order and/or guidelines are that it is optional for the employer, then I don’t think that there is any legal issue with them withholding the money. It all comes down to what the order is really saying.
A contrived example to underscore my possible reading of the order:
Personal attacks shall not be posted with respect to forum members.
The order is to the personal attack poster, the subject of the order is personal attacks and the scope the subject of the applicable attacks is another forum member.
So one could read the order:
(a) The deferral shall be made available with respect to any employee the amount of whose wages or compensation, as applicable, payable during any bi-weekly pay period generally is less than $4,000, calculated on a pre-tax basis, …
As saying that the order is to the entity making the deferral available (Treasury) and the subject is the deferral option for employers and the scope of the option is applicable employees.
Employers seem to be acting in good faith by expressing that they are not sure how to interpret the order. This would probably protect an employer from penalty if they choose to do nothing until the guidelines are published. If the guidelines show up at the eleventh hour— too late to change payroll, I think an employer would be protected from penalty.
Take the extreme case of an executive order changing withholding that is posted 1 hour before a payday. Technically an employer may not be in compliance, but highly likely a court would provide relief if the executive tried to asses a penalty.
For changes of this magnitude a reasonable time window for compliance is probably measured in weeks and months. And probably measured from the time the guidelines come out. Therefor my assessment is it is already too late to expect and enforce compliance for September wages.