IMPORTANT: Create a save file on your last visit to the Duke.
To get this achievement, you need to first complete the story mode at least once, this will unlock access to the Dragoon assault rifle.
After completing the game you'll already have a small amount of bonus points, enough for unlock "Dragoon" it the bonus shop. Then, load the save you did before and buy Assault Rifle for 200 Lei and "Recipe: Rifle Ammo" for 10000 lei. (If you dont have enough money just sell some of your guns).
Now all you need to do is craft some AR ammo, which then can be sold for 7 lei.
This is the only way to acquire a non-multiple of 5 lei.
Buy and sell items to the Duke in order to bring your money supply closer to the required number (777, 7777 or 77777), and then sell rifle ammo to get an achievement.
In the same way, it is possible to complete challenges for obtaining 54321 and 52911 lei.
(I'm not sure that in English version of the game it's called challenges.... sorry))