expression
even small mistakes can be costly
Proverb warning that a seemingly trivial error (here, a loss of just three ryō) can lead to significant trouble. Used to caution against carelessness.
「がったり三両」ということわざがあるように、ちょっとした油断が大きな損につながることもある。
As the proverb "gattari sanryō" says, even a small lapse in attention can sometimes lead to a big loss.