The following materials address the interesting question: If one agrees to do and does nothing, where does one do (or not do) it?...
Korfund Company, Inc. v. Commissioner
...The sole point of difference between the parties as to this income is whether it was earned from sources within the United States within the meaning of section 119 of the Revenue Act of 1938, and that, as already indicated, turns upon the source of the income derived from agreements not to compete with petitioner in the United States and Canada or give advice for the organization of, or to, a competitor.
The petitioner's contention is based upon the theory that the income was paid for agreements to refrain from doing specific things -- negative acts. No defaults occurred and during the period of compliance the promisors were residents of Germany. Petitioner's contention is that negative performance is based upon a continuous exercise of will, which has its source at the place of location of the individual, and that, as the mental exertion involved herein occurred in Germany, the source of the income was in that country, not in the United States where the promise was given. The respondent's view of the question is, in short, that, as the place of performance would be in the United States if Zorn and Stoessel had violated their contractual obligations, abstinence of performance occurs in the same place.
Taxation of International Transactions, Gustafson et al.