If, in connection with any charitable contribution, a liability is assumed by the recipient or by any other person, or if a charitable contribution is of property which is subject to a liability, then, to the extent necessary to avoid the duplication of amounts, the amount taken into account for purposes of Section 24357 as the amount of the charitable contribution—
(a)
Shall be reduced for interest (1) which has been paid (or is to be paid) by the taxpayer, (2) which is attributable to the liability, and (3) which is attributable to any period after the making of the contribution, and(b)
In the case of a bond, shall be further reduced for interest (1) which has been paid (or is to be paid) by the taxpayer on indebtedness incurred or continued to purchase or carry such bond, and (2) which is attributable to any period before the making of the contribution. The reduction pursuant to subdivision (b) shall not exceed the interest (including interest equivalent) on the bond which is attributable to any period before the making of the contribution and which is not (under the taxpayer’s method of accounting) includable in the gross income of the taxpayer for any taxable year. For purposes of this section, the term “bond” means any bond, debenture, note, or certificate or other evidence of indebtedness.
Source:
Section 24357.4, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC§ionNum=24357.4..