It is a technically wrong, but then very popular sort of spelling for 少【すくな】うして (canonically 少【すく】なうして).
After multi-syllable coalescence in the Late Middle Japanese period took away a considerable portion of phonological distinction manifested in the traditional orthography (based on Early Middle Japanese), people usually spelled words while inferring from the correspondence between their own pronunciation and the common pattern of spelling. That makes representation of word-internal or long vowels quite unstable in the actual world.
As a general tendency, vowel morae that are derived from grammatical alteration were likely spelled as if having //h// consonant (はひふへほ in place of あいうえお) regardless of etymology, probably after the quantitatively dominant ハ行四段活用 verbs.
用ひる (correctly 持つ + 率【ゐ】る > 用ゐる [[motiwiru]] > 用いる [[motɕiːru]])
書ひて (correctly 書きて [[kakite]] > 書いて [[kaite]])
来ねへ (correctly 来なへ *[[kənape]]? > 来ない [[konai]] > 来ねえ [[koneː]])
In the (modern) Western dialects, many conjugative endings are smoothed down to long vowels. The original form すくなくして has developed into スクノーシテ (//sukunoːsite//). In this case, such long vowels were often written like //oː// → あふ/おふ, //juː// → いふ, //joː// → えふ.
Despite historical incorrectness, some of those once widespread spellings are still used today (even after the orthographic reform) to set some ye olde atmosphere.
- かほり (correctly かをり [[kawori]] > かおり [[kaori]])
- どぜう (correctly どぢゃう [[dodjau]] > どじょう [[doʑoː]])
- せうゆ (correctly しゃういう [[(t)ɕaũ.iu]] > しゃうゆ [[ɕau.ju]] > しょうゆ > [[ɕoːju]])
why is it 少ふして and yet the transformation is not applied to 多く to become 多ふ as well?
Because the change in Classical adjective 連用形: 少なく > 少なう (> 少のう) never affected the 中止法 usage. In a nutshell, it only applies when the 連用形 form connects to (i.e. modifies or modified by) the right next word. 中止法 has become so rare in the spoken language that I don't know whether there are dialects that universally applies this mutation.
This is somewhat parallel to the split of verbal 連用形 (so-called masu-form and te-form).
酒を買い、家へ帰る。 (written language; 中止法)