UBER - I paid cash on non cash trip

I booked a trip for my mother and her sister on **th August ****. Since I was not with them, I told them the amount they I'll have to pay and attended my college. Later I discovered that we were supposed to pay Uber digitally as they automatically selects that option instead of cash. Now I cannot book any more rides till I pay them digitally also. Since we did pay the Driver partner it is very unfair that we have to pay again. I would really appreciate if Uber could verify the payment with their driver partner and nullify this charge on my account.