Viera spent three years with Beijing Guoan, having left Las Palmas, before returning to Spain.
The former Valencia attacker said: "When the offer from China appeared, there was not much to think about. It was an irrefutable contract.
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"I know where I come from and I come from very low. My mother has had to work a lot. It was a lot of money and I was going to leave a lot of money to the club. I am the player who has left the most money in the club.
"Both parties are going to win. Football is about making decisions. And that has been the best decision I have made in my career."