Must one repeat the prayers that were performed while there was a trace of urine on the underwear due to uncertainty about the cessation of urine drops?

If the presence of urine drops is a doubt after finishing the prayer, it should be disregarded. However, if their presence is certain, and it is probable that they exited after the prayer, then the prayer is valid.

However, if their exit is certain after ablution and before finishing the prayer, then the prayers performed without ritual purity must be repeated. If their number is unknown, then one should make up what brings certainty or strong presumption of having cleared one's obligation.

As for the water that touches clothes after purification (Istinja (definition)'), it is pure if it separated from the impurity after its removal and was not altered by it.