IS abducted the employees on Monday from Al-Badia cement factory outside the town of Dmeir, around 50 kilometres (30 miles) east of Damascus.
Local figures from Dmeir mediated a deal with IS on Friday to let the workers go free, according to the Syrian Observatory for Human Rights.
The Britain-based monitor said that in effect, some 170 workers would be freed as others had already managed to escape.
The Observatory later said that most of those kidnapped had finally reached their homes late Friday.
A military source told AFP that he saw dozens of cement workers pass through a nearby regime-held military airport.
"I am all right. Daesh (IS) let me go today and I went to the Dmeir military airport for a debrief and questioning, and then I went home," one man who had been kidnapped told AFP.
IS reportedly abducted the employees during a major offensive on Monday around Dmeir.
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The IS-affiliated Amaq news agency said in an online statement that it released about 300 of the workers, but that it would not free 20 men accused of belonging to a pro-government militia.
The statement said four of the cement workers were executed for being Druze, an offshoot of Islam considered heretical by IS.
"I heard that Daesh executed some Druze. My relative (there) is Druze and I'm scared that he's one of them. My mother is beside herself with worry and we're very afraid of receiving bad news," a Damascus resident told AFP.
Dmeir is divided between IS control in the east and rebel control in the west, but several key positions around it, including the military air base and a power plant, remain in government hands.
On Friday, warplanes carried out fresh strikes around the Dmeir military airport, the Observatory said.
IS has carried out mass kidnappings in previous offensives.
In January, it abducted more than 400 civilians, including women and children, as it overran parts of Deir Ezzor province in the east.
In northeast Syria last year, IS kidnapped at least 220 Assyrian Christians, many of whom have since been released through local negotiations.