Praise be to Allah.
Honouring one’s parents is one of the greatest means of attaining Paradise. What a great loss it is if a person’s parents reach old age and he does not make honouring them a means of entering Paradise. The Prophet (peace and blessings of Allaah be upon him) said: “May his nose be rubbed in the dust, may his nose be rubbed in the dust, may his nose be rubbed in the dust.” It was said: “Who, O Messenger of Allaah?” He said: “The one who lives to see his parents reach old age, one or both of them, and he does not enter Paradise because of them.” Narrated by Muslim, 2551. See also questions no. 22782 and 5326.
One of the ways of charity through which a person may seek reward with Allaah is to spend on his parents and take care of them with his own wealth. By doing that you will have the reward of doing charity in sha Allaah. The Prophet (peace and blessings of Allaah be upon him) said: “If a person spends on his parents, seeking reward, it will be an act of charity for him.” Narrated by al-Bukhaari, 55.
With regard to your question, if you buy goods for your father worth 150 riyals and he gives you 200, there is no sin in that, as he has voluntarily given you more than you paid. There is no riba in this case. Riba can hardly be imagined between a son and his father, and both of you are doing well in sha Allah. You wanted to give him the value of the goods and he wanted to give you something extra. We ask Allaah to reward you both and to bless you both in your wealth.