Or....

The manager instead of being an $$$ - should just have retained the voucher - and write her a receipt for the balance.... Yeah, there are rules - but these are guidelines, they are not the 10 commandments..

And then the manager would advise her to plan and spend the remaining Ksh. 4020/- the next time she comes so they can avoid complications...

This way, they will keep not one, but other customers..

Secondly, the manager should report to those who made the voucher software, and the necessary correction made and implemented, if not now, then in time.