The 22-year-old had arrived in the eastern city Wednesday for a medical ahead of inking a three-year contract for a four million euro ($4.4 million) deal which includes a buyback clause valid for two years, a source said.
Cash-strapped Lille, who Mothiba helped keep afloat last season in the top flight, notably with a final-day brace against Dijon, would also take half the transfer fee were Strasbourg to move him on during that time, the source told AFP.
Mothiba, on the South African Olympic squad in 2016, joined Lille in 2014 from the Diambars academy in Johannesburg and had a successful stint with second tier Valenciennes early last year to earn a place at Lille.
His early season form earned him a call-up to Bafana Bafana full squad this month but Lille chairman Gerard Lopez said the club was obliged to trim its squad.
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