Is it permissible to pray in trousers and boxers after drops of urine have dried on them, without changing them, knowing that these drops leave no trace of color or smell, and sometimes clean clothes are not available?

If, after urinating, you wait until the drops cease, then it is only necessary to wash the impure area on the body and the garment. It is not necessary to change the garment if it can be purified. If you are unable to wash it and cannot find a pure garment, then pray in it. It is more cautious to repeat the prayer after purifying it or finding a pure garment. It is recommended to place a tissue or similar item to prevent impurity from seeping onto the body and garment, then perform istinja' (ablution after relieving oneself), wudu' (ritual ablution), and prayer after the urine has stopped.