The Prophet sent Sa’d B. Waqqas on an expedition to al-Kharrar, to confront the hostile Quraysh. It is claimed by some that the Prophet sent Sa’d b. Abu Waqqas solely on this mission to raid a caravan. However, no such thing is reported in Ibn Ishaq. In fact, all it tells us is that the Prophet sent Sa’d B. Abu Waqqas with eight riders to a place called ‘Kharar’, but came back without fighting [Ibn Ishaq]:
“Meanwhile the apostle had sent Sa’d b. Abu Waqqas with eight men from the emigrants. He went as far as al-Kharrar in the Hijaz. Then he returned without fighting.” 
The Prophet must have received intelligence that the enemy, the Quraysh were there for him to send his Companions out on this expedition. We have to remember the small Muslim community were high on alert from the Quraysh since they declared war against the Muslims just before al-Kharrar incident.
One would suppose that once the Muslims fled from Makkah, from the years of persecution by the Quraysh against the Muslims that they wouldnt leave the Muslims alone. But that was far from their minds. These criminal hatched up plots to rise other tribes to drive out (or kill) the Muslims in Medina. Kitab al-Maghazi – Ma’mar Ibn Rashid:
The infidel Quraish wrote wrote to Abdullah ibn Ubayy ibn Salul and also to those members of the Aws and Khazraj tribes who were idolaters. This occurred while the Messenger of God resided in Medina but before the incident at Badr. The Infidel Quraish said,
“You have given shelter to our tribes men, and you remain the more numerous of Medina’s inhabitants. We swear by God that you had better either kill him or expel him, or else we will rally the Arabs to help us and march against you in our full numbers, slaying your warriors and ravishing your women!” 
“AbdurRahman ibn Ka’b ibn Malik reported on the authority of a man from among the companions of the Prophet (p): The infidels of the Quraysh wrote (a letter) to Ibn Ubayy and to those who worshipped idols from al-Aws and al-Khazraj, while the Apostle of Allah (p) was at that time at Medina before the battle of Badr. (They wrote): You gave protection to our companion. We swear by Allah, you should fight him or expel him, or we shall come to you in full force, until we kill your fighters and appropriate your women. When this (news) reached Abdullah ibn Ubayy and those who were worshippers of idols, with him they gathered together to fight the Apostle of Allah (p). When this news reached the Apostle of Allah (p), he visited them and said: The threat of the Quraysh to you has reached its end. They cannot contrive a plot against you, greater than what you yourselves intended to harm you. Are you willing to fight your sons and brethren? When they heard this from the Prophet (p), they scattered. …” (Abu Dawud Book 19, Hadith 2998)
Similar incidents are also reported in the Quran and other historical books that the Quraysh, threatened, stole, and even at times waged war when the Muslims resided in Madinah, these incidents occurred before the battle of Badr (Click here to read more: ‘Did Quraysh Persecute Muslims When They Fled To Madinah?’).
Although the report in Ibn Ishaq mentions nothing about a ‘caravan raid’, even if it is the case that their intention to set out on this expeditions to raid the Quraysh’s caravan. The Muslims had the right to take what was theirs, given the fact that the Quraysh robbed the property, houses and money from the Muslims in Makkah (Read about this here: ‘Property Of Muslims Stolen – Robbed By Quraysh In Makkah?‘).
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 Ibn Ishaq’s Sirat Rasul Allah – The Life of Muhammad Translated by A. Guillaume, page 286
 Ma’mar ibn Rashid: The Expeditions – An Early Biography Of Muhammad, page 67