I’m assuming that you’re using Transpose Master, and that both the legs and the overlapping garment are part of the same subtool. You appear to be using the Transpose action line version of Topological Masking. The problem with that in this situation is that topological masking masks according to topological continuity and you are trying to mask two separate overlapping mesh pieces.
In this case, it would be better to mask by drawing a masking marquee (ctrl drag with the default pen or masking lasso selected). This will “Mask through” the marquee, and any points that fall under it will be masked, masking both the leg and the garment at the same time.
It may require some precision to mask one leg and not the other since you have modeled the legs very close together. This is one reason it’s important to model a TPose in a relaxed neutral position with plenty of space in between legs, digits, arms, and body. Switching off perspective mode and masking from the front with the nice straight masking marquee you get from the default masking pen will probably be easiest. Remember that you can always adjust the placement of a drawn masking marquee by holding down the Spacebar and dragging while the marquee is drawn out and before releasing.
Remember also that the lower poly your TPose mesh is the easier it will be to mask points accurately, pose without distortion, and to repair any distortion that might occur.