Pronunciation of rebut

The "weak vowel merger": unstressed /ɪ/ often can be /ə/

In fully unstressed syllables, many English speakers don't have a clear distinction between /ə/ and /ɪ/. This has been called the "weak vowel merger". Most transcriptions of American English include the weak vowel merger, and generally transcribe the merged vowel as /ə/.

There are a few contexts where /ɪ/ is typically transcribed even in American English, though, like in the ending -ic as in the word static. I'm not to what extent this is predictable from the phonological context. In the context of Australian English, which is also supposed to show this merger, Cox and Palethorpe (2018) say "/ɪ/ is typically retained in suffixes like -ish, -ic, -ism, -ing, and /ɪ/ often still occurs when the following consonant is postalveolar or velar but this could possibly be considered allophonic (or free variation) e.g. paddock, stomach, beverage, manage" (p. 89).

Another complication is that some speakers who have this merger do use somewhat different qualities on average for word-final and non-word-final /ə/, and the word-final quality is retained even when a consonant like /z/ or /d/ is added by inflection. For speakers with this kind of "Rosa's" vs. "Roses" distinction, non-word-final schwa tends to be phonetically closer/higher than word-final schwa.

The complementary distribution of /ə/ and /ʌ/

In primary-stressed syllables, many English speakers don't have any distinction at all between /ə/ and /ʌ/. (Other speakers do report distinguishing stressed /ə/ and /ʌ/, e.g. in just "only" /ˈdʒəst/ vs. dust /ˈdʌst/.) If we assume that non-primary-stressed syllables containing /ʌ/ always have some kind of secondary or minor stress, then /ə/ and /ʌ/ are in complementary distribution for these speakers as long as we specify the stress pattern of a word.

This can be analyzed in different ways. One common way is to say that the phoneme /ə/ is phonologically restricted to unstressed syllables. But we could also say that /ə/ and /ʌ/ are not in fact distinct phonemes for these speakers.

Even if we don't actually adopt that viewpoint as a matter of technical analysis, the complementary distribution of /ə/ and /ʌ/ in the accents of these speakers means that we can unambiguously transcribe the phoneme /ʌ/ with the symbol "ə". Dictionary authors might choose to do this to reduce the number of unfamiliar symbols in their transcription scheme. I think that the symbol "ə" is more widely recognized than the symbol "ʌ", and most dictionaries are designed for a non-expert audience.