What I've found is that word-final /a/ in native Malay words and words up to the time of the Portuguese invasion (16-17th century) undergo reduction, e.g.:
/a/ → [ə] / __# Mata (eye) - [matə] Kura-kura (tortoise) - [kurə kurə] Gila (crazy) - [gilə] Jendela (janela, window) - [dʒəndelə] Kemeja (camisa, shirt) - [kəmedʒə] Tinta (tinta, ink) - [tintə]
Non-native words retain /a/, e.g.:
Sfera (sphere) - [sfera] Koma (coma) - [koma] Tuala (towel) - [tuʷala]
Surprisingly, the vowel in 'bola' (ball) does not undergo change.
Bola (ball) - [bola]
Based on the retention of the word-final /a/, we could arguably say that 'bola' is a foreign word.
If so, what might the native word for such a basic word be? If not, why is the vowel not reduced?