Is zakat obligatory on money that a person lent to his brother to help him get married, bearing in mind that the lender has inwardly forgiven the debt, and that the debtor intends to repay it but is in difficult financial circumstances, and is the zakat due on it every year, and is there a reward for the lender for doing so?

There is no zakat (definition) on lent money until it is received and reaches the nisab (definition), because it is a debt owed by an insolvent person. The intention to absolve the debt is not considered unless it is fulfilled. She will be rewarded for this, as it helps her brother maintain his chastity and strengthens family ties. This is also the reward for a goodly loan (qard hasan (definition)), which is like half of charity. Giving respite to the insolvent and overlooking his debt are among the reasons Allah will overlook His servant and grant him shade on the Day of Resurrection.