What was the reason for the argument?

Directions: Read the following passage and answer the question that follows.

Milan, the East Coast agent of the Inter-urban Express Coach, leaned over the counter of the express office and shook his fist. Mr. Gaikward, angry and red, stood on the other side of the counter, trembling with rage. The argument had been long and heated, and at last Mr. Gaikward had talked himself speechless. The cause of the trouble stood on the counter between the two men. It was a soapbox, across the top of which were nailed a number of strips, forming a rough, but serviceable cage. In it, two spotted guinea pigs were greedily eating lettuce leaves.
“Do as you like, then!” shouted Milan, “pay for them and take them, or don’t pay for them and leave them be. Rules are rules, Mister Gaikward, and Milan is not going to be called down for breaking of them.”