Why does the gear shift move forward from 1st to 6th gear when I take off ?

I have a new Polan Pro 42" PB175G42 riding mower purchased June 19th 2014. The belt came off so I took it to a authorized svc ctr and the tech replaced the belt and said all was fine he said "I drove it around the parking lot and it's ready" I started the mower and went to drive up on my trailer but the mower didn't move. The tech was standing there and after checking the mower said a linkage had broken. The mower was less than 60 days old so the repair was covered by warranty. When I went back to pick up the mower I started to drive across the parking lot to my trailer and noticed that when I selected 1st gear and took off the gear shift went all the way down to 6th gear but did not change speed, and when I applied the break it shifted back. Any forward motion and the shifter moved. The repair shop have had my mower for three (3) WEEKS and still have no idea how to fix the problem. Any ideas to get my mower back in the grass?