What you're looking for is the fact that word-final /ər/ may be lowered to a near-open central unrounded vowel [ɐ] in England, New Zealand and probably in non-rhotic US accents, certainly in AAVE (African American Vernacular English), and to a open central unrounded vowel [ä] in Australia. In other words, LETTER may surface as STRUT.
In North America word-final /ə/ may also be similarly lowered (but I think the most common output is an open-mid central unrounded vowel [ɜ], as that's the most common value of STRUT in USA), also in Afrikaans-influenced Broad South African accents (to a near-open central unrounded vowel), when spelled <a>. Diphthongs /eə̯ ɪə̯ ʊə̯/ may similarly, for some RP (mainly older), Australian and New Zealand English speakers have a lowered second vowel [eɐ̯ ɪɐ̯ ʊɐ̯]. There's a big variation when it comes to these diphthongs, and I'd have to write twice as big message to say enough about them. Just read Wikipedia and you'll get it quickly.
Received Pronunciation examples of lowered word-final /ər/:
employer /ɪmˈplɔɪ̯ə/, [ɪmˈpʰlɔɪ̯ɐ]
water /ˈwɔːtə/, [ˈwɔːtɐ]
better /ˈbetə/, [ˈbetɐ]
Note that first: such lowering is very rare at best (or probably inexistent) when the linking or intrusive /r/ is present, and second: you won't generally encounter lowered LETTER in other positions, unless you're listening to someone from Scotland. As far as I know, in Scottish English there's no phonemic /ər/, as it always merges with /ʌr/ to [ʌr]. But I have a feeling I'm somewhat wrong about Scottish English, so take it with a grain of salt.
General American examples of lowered word-final /ə/:
to (unstressed) /tə/, [tɜ]
the* /ðə/, [ðɜ]
comma /ˈkɑmə/, [ˈkʰɑmɜ]
*The version that doesn't rhyme with "bee".
RP examples of lowered /ə/ in centering diphthongs:
square /ˈskweə̯/, [ˈskweɐ̯]
near /nɪə̯/, [nɪɐ̯]
cure /kjʊə̯/, [kjʊɐ̯]
Bear in mind that even in narrow transcription you'll rarely encounter other symbols than the schwa, for two reasons: it's only a possible pronunciation, and phonemically those are /ər/ and /ə/.
Hope that helped, cheers mate.