The Costa Rica international joined the club from Dutch side FC Twente for £10.6m on the final day of the summer transfer window in 2011.
Ruiz has since made 51 appearances for the club, scoring eight times in the process.
But the 28-year-old has scored just once in 14 matches this season and is on the brink of a return to the Eredivisie.
"He did have a medical at the club on Monday and I can say that everything went well," Ruiz's agent Rene Vijt said.
"Now it is up to the two clubs before anything else can happen."
New Fulham head coach Rene Meulensteen admitted last week that he would allow Ruiz to leave this month.
"I wouldn't call his signing a mistake because at that time it was probably for Fulham the right choice and the right investment," Meulensteen said.
"Don't get me wrong, he is a very good player. I've come in, I've looked at the squad, worked with these players, I've picked my teams on what I think are the right reasons and motivations, but I can also understand Bryan's point of view."