Interest in credit card frameworks

Shalom,

Is there a prohibition of interest when opening the option of a fixed monthly charge (so‑called "Hachak" – fixed monthly billing) on a credit card, even if one does not use this option?
And what is the ruling if someone used the card beyond the set limit, intending to pay it all in that same month and not to need an interest‑bearing loan—is there any prohibition in this?

Shalom u’vracha,

In this case one may be lenient.

By “fixed monthly charge” is meant an agreement between the customer and the credit‑card company that they will not collect from him more than a fixed monthly amount, and if he purchased on the card more than that amount, the balance is carried over to the next month with added interest, of course.
There are those who enter into such an agreement not in order to benefit from the loan aspect of the credit line, but for other benefits of this plan (for example, the possibility of withdrawing cash from an ATM that will be debited from the account on the monthly billing date and not immediately). Now, the very framework and the carrying over to the next month with interest is, in effect, a loan with fixed interest (ribit ketsutsa), and it may not be permitted for purchases that are not for business purposes. However, one must consider whether it is permitted to enter this plan on condition that one does not buy beyond the limit, in which case there is no concern of interest at all. And this certainly should be permitted: so long as one has not spent above the limit of the fixed monthly charge, there is no loan at all upon which interest has been stipulated.
But one must consider the case where one did in fact buy more, yet intends, as a one‑time instruction, to tell the credit‑card company to collect the entire amount and not only the monthly ceiling. Here there is already a loan with stipulation of interest, but his intention is not to continue the loan in a way that will actually obligate him in interest. In the Gemara it is explained that the prohibition of “lo teshimon” (do not lend on interest) is transgressed by the lender from the moment of stipulation, even before the interest is collected.
Rabbi Akiva Eiger (beginning of Siman 160) is in doubt whether the borrower also transgresses from the time of the loan, or only from the time of payment of the interest; but Chavot Da’at (Siman 160, s.k. 1) simply rules that he does not transgress until the time of payment of the interest. Therefore one may be lenient in this case. As for the prohibition of “lifnei iver” and “lo teshimon” that the credit‑card company itself transgresses, one may say that, on their side, the credit‑card companies have a heter iska and rely on it (of course, all this is only in a case where the borrower owns an apartment/real estate; otherwise, a heter iska does not even begin to apply).