The London Stock Exchange is to buy a 60% stake in LCH Clearnet, a European clearing house.
The LSE will pay 366 million euros (£300 million), about 100 million euros below the original offer price.
The companies said the lower price reflected new European rules requiring clearing houses to hold more capital.
Clearing houses enable the trading of shares between two parties, charging a fee to guarantee the sales should one side default.
The British Office of Fair Trading approved the acquisition earlier this month.
By taking over LCH, the BBC reports the LSE should be better able to compete with the NYSE Euronext and Deutsche Borse, which own clearing houses.