Is it permissible for me to keep 50 riyals as profit from purchasing 200 riyals worth of credit with my own money, after my father gave me 50 riyals to buy credit?

The inquirer must purchase the recharge card for his father if he is acting as his agent in this matter, because an agent is bound by what he has been entrusted with and must act in the best interest of his principal. The recharge card grants the father a validity period that is not available when sending credit. However, if the father said to him: "Transfer or send me credit equivalent to its value," then in this case, the inquirer is a seller, not an agent, and there is no harm in doing what he mentioned in the question.