Excerpts from Books and Wikipedia
"Both men now wanted to break the treaty: al-Malik was ready to go back to conquering, and Justinian wanted to prove his worth as emperor. In late 692, al-Malik provided a useful excuse for both of them when he paid his tribute with Arab coins rather than Byzantine currency. He had been working for some time to stabilize the financial affairs in the Islamic empire by creating a standard Arabic coinage. Justinian II at once announced the coins counterfeit and unacceptable, and declared war. He had hired, to reinforce his own army, thirty thousand or so Bulgarian troops as mercenaries: “He levied 30,000 men, armed them, and named them the ‘special army,’” Theophanes writes. He was “confident in them,” certain he could meet al-Malik in battle with success.14 He was wrong. The Arab forces pushed into Byzantine land; when the armies met in 694 at the Battle of Sebastopolis, on the southern shore of the Black Sea, the Bulgarians cut and ran. Al-Malik had sent them money behind the scenes and promised them more if they would retreat once the battle began. The Byzantine troops were unable to hold the front without their allies and retreated."