What is required of one who seized wealth by force or stole it is to return the currency in accordance with its value on the day on which it was seized or stolen. If what was stolen was particular items, then the thief is liable for any drop in prices. In the case of other debts, there should be differentiation between small and great changes in the value of currency. If the change is small, and does not reach one third of the debt, then the same amount must be paid back, and no attention should be paid to the value of the loan when it was given. But if there was a big change, as much as one third or more, then an agreement must be worked out between the two parties to share the loss. If it is decided that payment is to be made on the basis of the value of the loan on the day when it was given, then it should be worked out according to the price of gold at the time when the loan was given, or according to the purchasing power of the currency at that time, but it should be paid back in a different currency than that in which the loan was given.

In principle, lending money is a righteous deed and is recommended (mustahabb), but in some cases it may be otherwise, so it may be obligatory (wajib), prohibited (haram) or disliked (makruh). Showing off, boasting and going to extremes in providing trousseaux for brides is, at the very least, makruh, and may be haram. If you know that the borrower intends to spend the money in a way that is makruh, then you should not help him in that. For more details, please see the long answer.