Orient set for big cash boost
The O's also stand to pocket a potential further ?170,000 in add-ons from the Championship club and will recieve 16.5% of any future sale.
Porter made 38 appearences for the O's last season, mainly as a substiute, after joining from non-league side Cray Wanderers.
The 20-year-old rejected the offer of a new contract at Brisbane Road and signed a three-year deal at Turf Moor in July.