sideline = dopunski posao; dopunski (dodatni) prihod (Vucicevic, "Pravni recnik").

Ovde bi trebalo imati malo siri kontekst da bi se tacno prevelo "payment-like sideliness".

Mozda neka "dopunska/dodatna isplata/placanje"?

Black's Law Dictionary:
side lines - 1) the margins of something, such as property;
2) a different type of business or goods than one principally enages in or sells;
3) Mining law: the boundary lines of a mining claim not crossing the vein running on each side of it;