The exercise book has one weakness
It remembers everything and tells you nothing. A page of names and figures cannot sort itself by age, cannot total itself honestly, and cannot remind anyone. So the debt that is easiest to collect gets collected, and the oldest quietly becomes a loss.
Age it, do not total it
A single number — customers owe me 40,000 — is not actionable. The same debt split into current, 7, 30 and 60+ days is. The 60-day column is where the money actually dies, and it is the column a paper book never shows you.
Remind on the cheap channel
A reminder that costs KES 0.05 gets sent. A reminder that costs KES 0.52 gets rationed. That difference decides whether you chase forty customers or two, so size the channel to the habit you want.
Write it down the same minute
Credit recorded at the end of the day is a guess. Credit recorded at the counter, against a named customer, is a ledger. The whole value comes from the timing.