Trey Burton had to pay money to catch passes for the Bears on Monday ... and he couldn't have been happier -- telling TMZ Sports he's donating tons of cash to a great cause this season!!
The ex-Eagle (and Philly Special star) says for every reception this year he'll give $1,000 ... and for every score, he'll shell out $2,500.
The cause?? Burton says it's all going to the International Justice Mission -- a global org aiming to stop the slave trade and sex trafficking everywhere.
Soooo ... with his performance Monday night against the Seahawks -- Burton had to pony up $5,500 ... and he couldn't have been more pleased to do it.
"The best $5,500 I've ever spent," he says. "Hopefully we can do a lot more!"
Trey tells us his season-long goal is to give out around $60,000-$90,000 -- and he hopes it inspires others to dip into their pockets to help as well.
"With how passionate of a sports town Chicago is," Trey says, "we figured this would be a really cool idea."