«رَقَبَةٍ مُؤْمِنَةٍ»
A believing enslaved person.
Raqabah is the noun being modified, and mu’minah is its adjective. The relationship shows that the required belief qualifies the person to be emancipated.
The adjective is grammatically feminine because raqabah is a feminine noun. This agreement does not restrict the verse to female persons; it follows Arabic grammar.
The syntax makes the legal point clear: the adjective qualifies the individual named in the ruling.