Sayyidna Jareer ibn Abdullah, Radi-Allahu anhu, once sent his servant for buying a horse. The servant made a deal for three hundred dirhams and brought the seller with him so he could be paid. Sayyidna Jareer ibn Abdullah, Radi-Allahu anhu, looked at the horse and realized that the seller had undervalued it. "Would you sell it for four hundred?" he asked. The seller agreed. "How about five hundred?" he continued his unusual "bargaining" and finally bought the horse for eight hundred dirhams. He was later asked why he did so. "The seller was not aware of the true value of this horse, " he explained. "I have simply given him a fair price because I had promised to Prophet Muhammad, Sall-Allahu alayhi wa sallam, to always be sincere and well-wisher for every Muslim."
(2:208) O Believers, enter completely into Islamand do not follow in the footsteps of Satan, for he is your avowed enemy. God demands that man should submit, without reservation, the whole of his being to His will. Man's outlook, intellectual pursuits, behaviour, interaction with other people and modes of endeavour should all be completely subordinate to Islam. God does not accept the splitting up of human life into separate compartments, some governed by the teachings of Islam and others exempt. (Tafheemul Quran)