WHOLE CHARACTER
斤
a market weight unit (half a kilogram); (斤斤) to fuss over small things; an axe (ancient use)
斤 is the market weight — one 斤 is half a kilo. In 斤斤计较 it means fussing over every little thing.
Taught as one written unit. ZDIC: 会意; an old axe — today mostly the market weight (半公斤 = 500 g).