a11 = a1 + 10d
a6 = a1 + 5d
a121 = a1 + 120d
a103 = a1 + 102d

(a11 - a6)(a121 - a103) = 360
(10d)(18d) = 360
180d^2 = 360
d^2 = 2
d = √2 or d = -√2

Since d is the common difference of an arithmetic progression, it must be a positive integer. Therefore, d = √2 is not valid, and d = √2 can be disregarded.

Therefore, the common difference of the arithmetic progression is d = √2.