The two clubs held talks on Monday night and agreed a compromise figure in excess of £8m, after Riera informed Espanyol he was only interested in moving to Anfield.
Everton had even made a higher offer for the 26-year-old.
However, Espanyol sporting director Paco Herrera insisted there were still some issues to be cleared up before a move could be completed.
"This is a sensitive time, and we need to make sure we reach a solution one way or another," he said.
"Everything is going well and a deal can be reached. The differences we have are ones we can solve and in the next two or three days there has to be a solution because there is no more time."