Narrated Anas ibn Malik ; Uthman ibn AbuSulayman: The Prophet (p) sent Khalid ibn al-Walid to Ukaydir of Dumah. He was seized and they brought him to him (i.e. the Prophet). He spared his life and made peace with him on condition that he should pay jizyah (poll-tax). – (Abu Dawud Book 19, Number 3031)
The Prophet (p) sent an expedition to Dumatul Jandal. The reason why the Prophet sent Khalid Ibn Waleed (ra) to the King, was because this King was in alliance with Byzantine (Roman) Empire. The Byzantine (Roman) Empire had declared war against the Muslim community. Which we have written about here and here. This King backed The Byzantine Empire financially and militarily.
Hence, Khalid Ibn Walid captured the King and brought him back to the Prophet, they made a peace treaty that the King will not attack the Muslim community, he agreed and signed a peace treaty.
Dr. Mustafa As-Sibaa’ie:
ARRIVING AT TABOOK
When the Prophet arrived at Tabook, he found not even trace of the large armies of Rome and of loyal Arab Christian tribes.
The Prophet certainly gave them ample time to bring their forces to Tabook, for he made camp there for twenty nights; but even had he stayed longer, they still would not have come. For the Romans were forced to show that they had no desire to enter into battle against the Muslims; and even Arab Christian tribes preferred to lay low while the Prophet was at Tabook.
As for the rulers of cities that were scattered all along the borders of Ash-Sham (Syria and surrounding regions), they decided to surrender peacefully, to sign peace treaties, and to promise to pay the Muslims a mandatory head tax the Jizyah; for example, the king of Ailah sent gifts to the Prophet a while mule and a robe and agreed to pay the Jizyah.
Ukaidir, the king of Daumatul-Jandal, also signed a treaty with the Muslims and agreed to pay the Jizyah tax.
But his case was different in that he did not voluntarily go to the Muslims; instead, he made up his mind to make peace with them after he was captured by them.
The Prophet had sent out a unit of four-hundred and twenty horsemen to Daumatul-Jundal. Headed by Khaalid ibn Waleed, the men of the unit managed to capture Ukaidir ibn Abdul-Malik Al-Kindee, who was out of the city on a hunting excursion. It was after Ukaidir was taken to the Muslims that he agreed to pay the Jizyah tax.
Shaykh Allamah Shibli Nomani:
At Dumat Al-Jandal he presented a white donkey which brought to him the gift of the Prophet’s garment in return. The Christians of Jarba and Adhruh attended likewise and consented to the levy of the Jizya. At Dumat al-Jandal (also pronounced as Daumat al-Jandal), which is five stages from Damascus, there was an Arab chief, Ukaidir by name, who owed allegiance to the Roman Emperor.
Khalid Ibn Walid was despatched with four hundred and twenty men to subdue him. Khalid made captive, and later on released him on condition that he would personally appear before the Prophet (p) to settle terms. Accordingly, he arrived accompanied by his brother and was promised protection. 
Even before this incident, this same King and its people had caused trouble. They harassed traders. Besides that, it is reported by many sources that there was a large army forming in order to attack the Muslims. Hence, the Prophet (p) sent out a delegation to deal with them.
Ameer Ali Syed:
Of this nature was the expedition against the Christian Arabs of Dumat al-Jandal (a place, according to Abulfeda, about seven days journey to the south of Damascus), who had stopped the Medinite traffic with Syria and even threatened a raid upon Medina; these marauders, however, fled on the approach of the Moslems, and Mohammed returned to Medina, after concluding a treaty with a neighbouring chief, to whom he granted permission of pasturage on the Medinite territories. 
Shaykh Allama Shibli Numani:
GHAZWA DUMAT AL-JUNDAL
In the month of Rabi al-Awwal, of the same year, the Prophet (P) was informed of the infidels mobilising a huge army at a place known as Dumat al-Jandal. He marched out with 1,000 Companions, and again the enemy took to flight on hearing of his approach. 
Shaykh Allama Shibli Numani:
Sariyah Dumat al-Jandal (also read as Daumat al-Jandal): In the year 4 A. H., 626 C,E., news was brought to the Prophet (P) that there was a concentration of men at Dumat al-Jandal, which lay fifteen stages from Medina in the direction of Syria; and that this band was harassing traders.
The Prophet (peace and blessings of Allah be upon him) himself led a force against them. The men escaped away, but the Prophet (peace and blessings of Allah be upon him) stayed there for a number of days and detailed small parties of armed men to various places all over the neighbouring region. 
Kitab al-tabaqat al-Kabir – Ibn sa’d:
Then (occurred) the Ghazwah of the Apostle of Allah, to Dumat al-Jandal in Rabi al-Awwal, after the commencement of the forty-ninth month of hijrah. They (narrators) said: (The news) reached the Apostle of Allah, that a large number of men had assembled at Dumat al-Jandal and that they treated cruelly the camel-riders when they passed by them, and intended to attack al-Madinah. 
THE EXPEDITION TO DUMAT AL-JANDAL AND OTHER EVENTS
According to al-Waqidi: In this year he mounted an expedition against Dumat al-Jandal in the month of Rabi’i. The reason for it was that word reached the Messenger of God that a host had assembled there and had approached his territories; so the Messenger of God mounted an expedition against them and reached Dumat al-Jandal, but he had no clash with the enemy.
He left Siba b. Urfutuh al-Ghifari in charge of Medina. According to Abu ‘Ja’far [al-Tabari]: In this year the Messenger of God made a truce with Uyaynah b. Hisn that the latter might pasture his herds in Taghlaman and its vicinity.
According to Muhammad b. Umar [al-Waqidi]-Ibrahim b. Jaf’ar- his father [Jaf’ar b. Mahmud]: This was because Uyaynh’s lands became a truce, so that Uyaynah might pasture his herds in Taghlaman as far as al-Marad. The land there had become lush with pasturage because of a rain cloud that had arrived.
The Messenger of God made a truce with him that he might pasture his herds there.
According to al-Waqidi: In this year the mother of Sa’d b. Ubadah died while Sa’d was journeying with the Messenger of God to Dumat al-jandal. 
 The Life Of Prophet Muhammad: Highlights And Lessons by Dr. Mustafa As-Sibaa’ie, page 1439 – 1440
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a), volume 2, page 238
 The Spirit of Islam Or the Life and Teachings of Mohammad By Ameer Ali Syed, page 72
 Sirat -un- Nabi By Shaykh Allama Shibli Numani, volume 2, page 99
 Sirat -un- Nabi By Shaykh Allama Shibli Numani, volume 2, page 263
 Kitab al-tabaqat al-Kabir By ibn sa’d, volume 2, page 76
 The History of Al-Tabari: The Victory of Islam: Muhammad at Medina, volume 8, page 4 – 5