The affix ap comes Invariably after the verb paṇ 'to barter' when the sense is that of 'measure'.,
The word नित्यं 'invariably' is used in the aphorism in order to stop the anuvritti of the word optionally. Thus मूलकपणः 'a handful of radishes'; शाकपणः 'a handful of vegetable'. These words secondarily have come to mean 'a measure equal to a handful', irrespective of any thing.
When not meaning measure, we have पणः 'trade'.,