I understand the issue of being forbidden to pay or recieve interest on a loan between private individuals. Is the same issur true when it comes to linking a Shekel loan to the dollar, i.e. money is added onto the loan amount if the principal is devalued with respect to the dollar. Will the answer be different if there was an agreement between the parties to this linkage before the loan was made without knowling the Halachic ramifications?

If the Shekel loan is linked to the dollar in both directions, that is: if the value of the dollar goes up more Shekalim will be returned but if the value of the dollar goes down, fewer Shekalim will be returned, there is no interest, but rather the loan was of dollars, but the currency of payment is Shekalim. If the loan is linked in a way that the repayment must go up, but will not stay the same or go down, a Heter Iskah should be signed. In regards to the basic prohibitions, there is no difference if the Halacha was known at the time of the loan.