If you are paying your former spouse for his 50% interest in community income earned by you while married, that reduces your taxable income but it does not reduce your self-employment income for purposes of calculating self-employment tax. I previously explained how to report this on your individual income tax return. Self-employment tax is paid by the taxpayer who earned the self-employment income. You indicated that you earned the Schedule C income and, therefore, the self-employment tax is reported on your Form 1040.
See Internal Revenue Code Section 1402(a)(5)(A).
https://www.taxnotes.com/research/federal/usc26/1402?highlight=%221402%22
Emphasis added below.
"(5) if--
(A) any of the income derived from a trade or business (other than a trade or business carried on by a partnership) is community income under community property laws applicable to such income, the gross income and deductions attributable to such trade or business shall be treated as the gross income and deductions of the spouse carrying on such trade or business or, if such trade or business is jointly operated, treated as the gross income and deductions of each spouse on the basis of their respective distributive share of the gross income and deductions"
