Is it permissible for me to buy gold for someone for 12,000 Saudi Riyals, then sell it to him for 20,000 Saudi Riyals in monthly installments?

It is not permissible to sell gold for banknotes unless it is hand-to-hand, without any delay in receiving the price, even for a short time. This is based on the saying of the Prophet (peace be upon him): "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt – like for like, equal for equal, hand-to-hand. If these types differ, then sell as you wish, provided it is hand-to-hand."