Question:
If I gave someone a punch on my bus card last week and this week the bus fare went up, can they give me back a punch this week?
Answer:
If the arrangement was that you would pay back cash, then one may not pay back more than the original value, nor may one return a bus card with one “punch” left if this is now worth more, as this would amount to paying interest.
However, if the arrangement was that one would pay back with a punch on the card, then it is permissible (even if the price increased in between) as long as the price was fixed at the time of the “loan” and was not due to increase in the immediate future.
If, however, the price increase was expected, one may not lend the punch of the card with intention to return the same thing, since in between it will increase in value and one would be paying interest on the loan.
In such circumstances, one should agree at the outset either to repay the loan of the punch in cash at the value of the punch, or to repay the loan in cash at the value of a single journey (although the lender bought this journey for a cheaper price since he bought a card which offers a reduction if one buys several journeys together, nonetheless he’s entitled to charge others for each punch the price of a single journey).
However, if at the time of the loan of the card the borrower had a similar card in his possession (even if it is not on his person at the time) one may make an arrangement to repay the loan with a card that has a punch available, even if the price is due to increase.