my parents keep taking 5's 10's etc as extra when they borrow some i approve of.

They ask to borrow money.. which I do usually get paid back after a reminder of the 10th time.. but recently ive been noticing cash gone.. 5's, 10's etc... one exmaple is, They took 140$ for a phone battery which i allowed 140$ ONLY to be taken and 150$ was missing? THEY never gave me the change NOR told me this.. what do I do to prevent this?