The system actually works in a way, that an item, that is added as merchandise, will grant its bonus sesterce with the next "tick(s)" (end of cycles).
For example: the timer (when hovering over the building) indicates that the current cycle ends in 5 minutes ("+x in 00:05:00"). If you add a beetroot to the merchandise slot now, you will gain the additional 90 sesterce in 5 minutes when the current cycle is over.
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