The 28-year old Colombian centre-back joined the Union from homeland club Santa Fe in 2011 before he returned to Bogota for a season-long loan in 2013.
It is believed that Valdes will again be loaned out by the Union with a spokesman making that suggestion to MLSsoccer.com.
"We can confirm that the Union have a six-month loan offer from San Lorenzo," the spokesperson said. "Nothing is finalized, though."
Valdes added to the Argentine press: "For now, there's some verbal agreements, a pre-contract, that surely will be finalized today or tomorrow.
"The idea was to get out in front of it and travel and do the physical examinations and everything, and hopefully in the next day or two, the clubs can come to an agreement on the remaining details."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball