This law is largely created by the courts, but some aspects have been codified into the Internal Revenue Code.
The claim of right doctrine, as it dictates whether the "right" to the income subject to a contingency that may take the income away is taxable in the US, originated in the North American Oil Consolidated v. Burnet decision.
In other words, A taxpayer must report the receipt of income for the time that she or he has control over it.
If a taxpayer ends up having to return the income recognized under the claim of right doctrine, then the taxpayer may receive a tax credit for that amount according to the Internal Revenue Code, if such a credit is a greater tax benefit than a deduction.
[2] The courts limited the claim of right doctrine and will not allow the IRS to make the taxpayer recognize income if there are significant restrictions on the taxpayer's disposition of the income.