The price of setting up a refrigerator is X.

The annual salary is 8400+X.

The monthly salary is (8400+X)/ 12, which is 700+X/ 12.

Seven months' salary is (700+X/ 12) multiplied by 7, which is 4900+7x/ 12.

The actual salary is 3900+X.

The result is 3900+x = 4900+7x/ 12.

Solving equation = 2400 yuan