What is the ruling on purifying trousers that have been touched by the feces or urine of a dog while wet: Is it obligatory to wash them seven times with earth, or is it sufficient to wash them once with soap without earth?

As long as you are afflicted with waswasa (obsessive doubts), it is permissible for you to adopt the easiest opinions. Some scholars do not stipulate seven washings and the use of dust except for containers licked by a dog. Some scholars are of the view that soap takes the place of dust in removing the impurity of a dog, and this is the relied-upon position in the Hanbali school. So, if the impure spot is washed with water and soap, and water is poured over it seven times, that is sufficient for its purification. Impurity does not transfer from an impure pair of trousers to whatever it touches, except under specific conditions.