PARIS, Dec 9 (Reuters) - French retailer Casino (CASP.PA) said on Thursday it had completed the sale of a 3% equity stake in commercial property company Mercialys, cashing in 24 million euros ($27.10 million). The sale, made through a total return swap (TRS) of maturity March 2022, reduces Casino"s stake in Mercialys in terms of voting rights to 16.9% from 16.9%, the statement said. ($1 = 0.8857 euros)