Helping a man to hold a wedding

Q 2: There is a tradition that the newly married couple should be given some money. It is then expected that the person who pays the money takes it back when he gets married. However, there should be some sort of increase on the paid amount. For instance, it may happen that a man gives five-hundred Riyals to someone as financial aid on the occasion of his son's marriage. One month later, the person who paid the money or his son gets married. In this case, the recipient of the five-hundred Riyals has to repay that amount and pay another five-hundred Riyals or less. The point is that some extra money should be paid over the amount he first received. One should not return the exact amount he was given at first; rather, an extra sum should be added. Returning the exact amount without any increase will cause ill feelings. This tradition is common among the people of the south, particularly on the occasion of marriage and helping in building new homes. Is it permissible to pay this financial aid that is repaid in excess? Does it comprise an element of Riba (usury) which is principally established on dealings with loans given on a conditional increase? The extra amount is somewhat mandatory; it must be paid even if a person has to borrow it. The same thing happens with women. A woman sends a specific amount of money to another woman, and in the former's wedding, the latter has to repay the money with an increase. This increase looks like a condition of repayment. The period of repayment may vary; a person who was given the money would marry, and the following week (Part No. 19; Page No. 139) the person who gave him the money may also marry. Thus, the latter should pay the former this amount with an increase one week later. It may be one month, or one year; according to the person's situation. What is the Shar`y ruling on this? May Allah protect you.