The torture increased, particularly in public to add humiliation to physical abuse. However, Abu Bakr bought Bilal and set him free to enjoy a peaceful life. He remained close to Abu Bakr, the closest companion of the Prophet (peace be upon him). When the Muslims of Makkah immigrated to Madinah, Bilal was very close to the Prophet. He was the one who regularly called for the prayer, as he had a melodious voice. He functioned in a way, which is equivalent to his being the Prophet's personal assistant and treasurer.
One day, Bilal looked at the dates the Prophet had but did not like them. They were of low quality and he thought that they were unsuitable. So he went to the market and exchanged them, bringing a better type for the Prophet.
His action led to a clarification of important Islamic rules concerning usurious transactions, as this is clearly explained in the following Hadith reported by Abu Saeed Al-Khudri: Bilal brought the Prophet some Barni dates. The Prophet asked him, 'Where has this come from?' Bilal answered, 'We had low quality dates. I sold two measures of that for one measure of this type to give to the Prophet'. The Prophet said, 'Ow! Ow! Absolute usury. Do not do that. If you want to buy these dates, sell yours for something else and then buy this with what you have received.' (Related by Al-Bukhari).
The rule that this Hadith sets is that a straight exchange of two different quantities of the same type of goods is usurious. As such, it is forbidden in Islam, even though the difference in quantity is based on the fact that they are of different quality.
When dates are exchanged for dates, or rice for rice, or apples for apples, the two quantities must be the same, e.g. one kilo for one kilo. It is not permissible to exchange two measures of a low quality product for a lesser measure of a higher quality type of the same product.
The Barni dates Bilal brought the Prophet were of the best quality. They were perhaps worth more than double the price of the low quality ones he had. Yet the Prophet insisted that such an exchange must be on the basis of equal weight or measure. His displeasure was expressed in terms of physical pain, 'Ow! Ow!'.
He described the transaction as 'absolute usury' to indicate its prohibition, as all usury is forbidden in Islam. He then instructed Bilal to return the dates and take his original ones. He should sell them either for cash or an exchange for something other than dates. He could then buy the high quality dates for cash or for what he received.
Al-Bukhari has listed this Hadith under the heading of 'Agent's Action', because the dates belonged to the Prophet and Bilal was acting on his behalf.
Thus, the Hadith tells us that an agent, or a person acting for another on the basis of a power of attorney, cannot do a prohibited transaction.
If he does, the transaction is illegal and invalid. The person on whose behalf it is undertaken is entitled to return the goods and get a refund. This is what the Prophet instructed Bilal to do.