Could you please explain the phrase in hafrashas trumah and maaser where we say hu vechumsho mechulal al haprutah. Where is there a chomash ? Chomesh is when you are mechalel one hundred dollars worth of produce on 125. Here, we are mechalel the 100 dollars worth of produce on a prutah ?
After separating the maaser sheini, we are mechalel it onto a peruta worth of local currency. This means even if the value of the maaser sheini is really worth much more than that. For example if someone separated maaser sheini from $10 worth of apples, the value of the maaser sheini is now $1. That dollar can be moved to a five cent coin. Additionally when a person is exchanging and moving his own maaser sheini, he has to add a chomesh – a fifth (which is really 25%) therefore he now is moving $1.25 worth of value onto the five cent coin. The extra chomesh that he has to pay is his personal obligation to add to the maaser sheini. So no matter where the “money” of the extra chomesh is, whether it is in his pocket or part of the fruits, he is putting it’s value on the peruta.
I hope this clarifies things.