ولو دفع إلى شخص ثم علم أنه عبده أو مكاتبه لم يجز في قولهم جميعا
Translation: If the recipient later turns out to be the payer’s own slave or mukātab, the payment does not count according to all of them.
The text explicitly says “according to all of them.” Unlike the preceding debated mistake cases, there is agreement that giving to one’s own enslaved person or mukātab does not discharge zakat.
The underlying ownership-transfer issue is decisive.