Can a Jewish-owned business that extends interest-free house account/credit, billable on a monthly basis (as per signed agreement), demand early payment? Would this be considered taking interest, as the borrower will lose two weeks of using their money (say investing the cash), while the store could themselves place the early payment into an investment that earns them money? The consumer loses two weeks' worth of investment, while the store gains the same. Is this considered a prohibited form of interest?
When a loan is given and a time is set for the return of the loan, the lender may not demand the return of the loan before the set time has arrived (Choshen Mishpat 73, 2). However, if the borrower wishes to repay the loan before the time has arrived he may do so (Choshen Mishpat 74, 2). This is the general rule (albeit there are certain special circumstances where the halacha is different - see in the previous sources and the commentaries for the details). Either way, early payment of the loan is not considered interest, as interest only applies where the lender gains more than the repayment of the loan. Early repayment is not considered a gain, as if he wouldn't have given the loan he would have had the money the whole time. Thus he has not gained anything from the loan. The only issue is whether the lender is entitled to the money before the time has arrived, and the basic rule (barring special circumstances) is that he is not, but if the borrower agrees to pay then he may do so.