Is the prayer of one who discovers impurity on his clothes after performing it valid, and is it obligatory to repeat it, and is it permissible to wipe away the impurity with water when unable to change clothes?

There is no need to repeat the prayer if it is probable that the impurity occurred after performing it with purity. However, if there is an indication that the impurity existed before the prayer, scholars have differed on the necessity of repeating it. Duha prayer is not obligatory in the first place, and one is not required to repeat it. It is sufficient to wash the site of impurity with water until its substance, color, and odor are removed; wiping is not sufficient.