I will ask the question using the following scenario:
Reuvan wrote a sefer that he was charging for it 50 Shekels. Came along Shimon who wanted to buy the sefer. At that time he didn't have money on him so Reuvan agreed to give him the sefer now and that he could pay later. When Shimon came to pay a few days later he decided he wanted to give Reuvan a 100 shekels for the sefer instead of 50. His intention in giving it was to be for his (Reuvan's) benefit since he is a Talmud Chacham and wants to give him more for this sefer. Is there any problem of ribis with such a situation?