US investors have agreed to increase their stake in a deal to buy the Chicago Stock Exchange (CHX) to prevent the politically sensitive bid, led by China-based investors, from falling apart, sources familiar told Reuters. A sale would mark the first time Chinese investors have been direct owners of a US stock exchange. Worth up to $25 million, the deal has drawn the attention of several lawmakers who have argued that despite its small size, a sale could expose the US financial system to undue influence from the Chinese government. The future of CHX has been in limbo since August, when the US Securities and Exchange Commission stayed a decision by its staff approving the sale to a consortium of 13 investors, led by China-based Chongqing Casin Enterprise Group. The planned sale, which was announced in February 2016, was cleared in December by the Committee on Foreign Investment in the United States, which scrutinizes deals for potential national security concerns.