A. M. Fehn considers the source meaning "to become rich"

Proto-Slavic *bogъ: Old East Slavic богъ (bogŭ). Old Ruthenian богъ (boh), бѡгъ (bôh), бугъ (buh), Belarusian бог (boh), Carpathian Rusyn бог (boh), буг (buh), Ukrainian бог (boh), біг (bih), бі (bi), Russian бог (bog). Old Church Slavonic богъ (bogŭ), Bulgarian: бог (bog), Macedonian бог (bog), Serbo-Croatian бо̑г, bȏg, Slovenian bọ̑g, Old Czech: bóh, Czech bůh, Old Polish bóg, Polish bóg, Silesian bōg, Polabian büg. Kashubian bóg, Slovincian bóg, Slovak boh, Lower Sorbian bog, Upper Sorbian bóh

Presumably, Slavic *bogъ had the meaning 'share, wealth, luck' (compare cognates Sanskrit bhaga 'happiness, welfare, prosperity', Avestan baga 'share', Tocharian A pāk part, portion, share'). It have been displaced by 'god' under Iranian influence and is attested mainly in compounds and derived terms: *bogatъ 'rich', *nebogъ 'poor, unfortunate', *ubogъ 'poor, *bogačь 'wealthy man', *bogatbstvo 'wealth, riches, fortune, treasure, (figurative) abundance'.

Meaning 'god' can be a semantic loan from from Iranian (Skythian?) source from Proto-Iranian *bagáh 'god': Avestan baγa. baga 'share, (favourable) lot', Bactrian βαγο (bago /⁠baγ⁠/). Khwarezmian ʾβγyk /⁠*aβγīk⁠/ 'little idol, doll, puppet', Sogdian: (/⁠βaγ⁠/), Early New Persian ḇaġ, Persian faġ (), Parthian ⁠bag (early), baγ (late), Old Persian b-g /⁠bagaʰ⁠⁠/ 'god', Middle Persian ⁠bag (early), bay (late)⁠/, 'lord, (earthly) god; divine'. From Proto-Indo-Iranian *bʰagás 'portion, part, share, allotment', 'fate, destiny', '(religion) "the dispenser"; a god'

Фасмер I, 181—182, ЭСБМ 1, 366, ESUM 1, 219

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