The possessor’s grammatical gender is neutral, the corresponding possessive pronoun is sein/seine/sein (which is identical to the pronoun for a possessor of male grammatical gender) – no matter what the possessor’s biological gender. However, sometimes the possessive pronoun corresponding to the possessor’s biological gender is used in a constructio ad sensum, especially if the biological gender is known, which is the case for the word Mädchen, for example.
Das Mädchen spielte mit seinem Auto. – The girl played with her car. (correct)
Das Mädchen spielte mit ihrem Auto. (constructio ad sensum)
Ich nahm das Brot und schnitt durch seine Kruste – I took the bread and cut through its crust.
Fascinatingly, I have never seen a constructio ad sensum, if Person, which is of female grammatical gender, is used to refer to a clearly male person.
Die Polizei sucht eine unbekannte männliche Person, die ihre Haare grün gefärbt hatte. – The police is looking for an unknown male person, who dyed his hair green.