qgis - Map scale vs. map extent vs. map canvas size in Print Map Composer

I am using QGIS 2.0 and when I try to create my map in the print composer, I cannot make the scale and the extent the way that I need it.

I need my map to be at the scale of 1:10 000. However, the map area is quite large and I need a specific x min and y max in the extent (the x max and y min do not matter). I also need a full A4 size sheet of paper.

When I enter the scale in, the map extent is not where I need it. If I change the extent, it either changes the scale of the map or changes the size of the map canvas in the print composer. When I change either one of those back, the scale changes and so on and so forth.

Is there some way to lock the scale and then just move the map image inside the map canvas?