The ex-Brentford center-back was called up to the England squad for the first time on Thursday for friendly matches against Netherlands and Italy this month.
Tarkowski left the Bees for Turf Moor in 2016 in a £3million deal that included an additional payment once the defender makes an international appearance.
And the Championship club will receive a further £500,000 if the 25-year-old makes a competitive appearance for England in a major tournament or a qualifier.
Gareth Southgate has scouted Tarkowski extensively over recent weeks and the feeling is that he could make the plane to Russia if he impresses when he gets his chance.