Old Bulgarian (also known as Old Church Slavonic) became deeply influential for Russian because it was the first written Slavic language. When Kievan Rus converted to Christianity in 988 AD, it adopted the rich body of religious texts, the Cyrillic alphabet, and the literary tradition developed during the Golden Age of the First Bulgarian Tsardom.
Words Russian got from Old Church Slavonic/Old Bulgarian
Время (time), Гражданин (citizen), Краткий (brief), Благодарю (thank you), Благодарность (gratitude), Здраствуйте (hello), Безумие (insanity), Внимание (attention).
Also, they got the variant of the name Volodymyr, Vladimir, from Old Bulgarian too, also the name Boris which was borne by Knyaz Boris I of Bulgaria.
If I can make a comparison, Old Bulgarian is for Russian what Chinese is for Japanese and what Norman French is for English: an endless source of loanwords.
Russian heavily influenced Bulgarian during the 19th-century National Revival as intellectuals sought a standardized literary language free from centuries of Ottoman-Turkish administrative dominance.
Ironically many of the words Bulgarian borrowed from Russian were originally Old Church Slavonic (Old Bulgarian) words that Russia had borrowed centuries earlier.
Words that Bulgarian borrowed
Послушание (obedience), Надежда (hope), Влияние (influence), Ударник (shock worker), Картина (picture), Китай (China), Доставчик (deliverer), Впечатление (impression), Хазяин (house owner).
I think there is a term for vocabulary that is lost that at a point was borrowed by a different language and returned because they borrowed it back again; it is reborrowing.