Volume 3, Book 34, Number 419 :
Narrated by ‘Abdur-Rahman bin Abu Bakr (radiallaahu `anhumaa)
We were with the Prophet (sallallaahu `alayhi wasallam) when a tall pagan with long matted unkempt hair came driving his sheep. The Prophet (sallallaahu `alayhi wasallam) asked him, “Are those sheep for sale or for gifts?” The pagan replied, “They are for sale.” The Prophet (sallallaahu `alayhi wasallam) bought one sheep from him.
Ibn Battaal said,
Trade with the disbelievers is permitted, but you cannot sell things to people at war with the Muslims, thereby helping them against the Muslims.
Ibn Taymiyyah said:
The principle is: people are allowed to do whatever they need to do, so long as this has neither been prohibited by the Qur’an nor Sunnah. This same principle applies inversely to acts of worship; by which one intends to draw near to Allah: such acts are invalid without authority from either of these sources. The aim of worship is only to draw near to Allah. For religion is what Allah has sanctioned, and the prohibited is what Allah has forbidden; in contradiction to those whom Allah has condemned, who forbade things which He had not forbidden, associated partners with Him without any instruction, and invented acts of worship without His Authority.
Starting from this rule, and relying upon the text of the Qur’an, the Sunnah, the practice of the Prophet, his Companions and the Imams of the Ummah, we can say, dealings with the disbelievers for purposes of trade and so on cannot be considered evidence of alliance. In fact, trade with them is allowed. When Ibn Taymiyyah was asked about trade with the Mongols he said, “Whatever trade is allowed with others is also allowed with Mongols. That which is not allowed with others is not allowed with Mongols. You may buy goods they produce, horses and so on from them, just as you may buy such things from Bedouins, Turks or Kurds. You may likewise sell them food, clothing and such things as you are allowed to sell to others.