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.