Austin Frakt via HealthReformGPS noting that the pay-for in the doc fix bill actually 'pays for' an indeterminate amount of money (Senate Finance committee says $19.1 Billion over 10 years) because the Secretary of HHS has the final authority to set the payback amounts for persons who get premium subsidies that are too large due to income increases during a year in which they got such subsidies. Amounts noted are maximums.
A separate source I talked with confirmed they thought that the above interpretation was correct and the amount of 'pay for' in the doc fix depended upon what the Secretary did. Further, they said that the language in the doc fix bill would have likely been in a conference committee bill reconciling the House and Senate reform bills if there had been one. It obviously wasn't a part of the reconciliation bill, however.
Update: Here is another take, discussing the undermining of the ACA (h/t @HEALTH_NOTES) by reducing subsidies in a way that would be a hardship to folks who got a better job during the year. This take assumes that the pay back amounts are definitely the amount that will be charged; the take from HealthReformGPS written by Sara Rosenbaum, and what Austin was commenting on. assumes the amounts specified in the doc fix are ceilings, or maximums and that the actual amount of pay-backs that people will have imposed on them if they get a better job is to be determined by the Secretary of HHS.
If the payback amounts set forth in the Medicare Extender/doc fix bill are maximums, then the degree to which this bill undermines the ACA is really dependent upon what pay back levels the Secretary of HHS sets. The general principle that taking bits of money out of the ACA will undermine it is correct, but it seems as though this bill may not do so much of that in reality.