If uber really want to go with cash method, should have provide coupon to the rider.

The rider can purchase coupon of any amount, the driver can sell these coupons when passenger is onboard.

The coupon can be scanned using qr code or barcode to deduct the amount until the particular coupon is depleted the value.

With coupon, u can insist the rider to provide a valid coupon before u can take him to the destination.

The workflow would be this:

1) you get a rider request, rider hop on to your car
2) you ask the rider for a valid coupon, if rider have none, they have to buy from the driver
3) rider then buy the coupon, be it rm5, 10, 20.
4) The coupon would have barcode or qr code to scan by smart phone.
5) the coupon is scanned at the start of the trip.
6) at the end of the trip, the coupon is scanned once again, the fare amount is deducting from the value of the coupon.

It's better than payment via cash at the end of trip:
1) you can validate a rider before you can fetch him to the destination
2) driver's earning is guaranteed

Of course, this requires the uber app to change as well, unless there's a third party app that can intervene.

I am considering to build 1 to test.

No identification is required to purchase the coupon, as long as the rider purchases from driver, he can use the coupon anywhere he want. Just like how one purchase bus ticket to travel from 1U to Genting.

For the coupon, the best possible solution woule be using e-coupon, provided the uber app is capable of generating it.

There's still many aspects to consider the workflow of e-coupon, how to generate validate a coupon and etc. For the time being, it's the technicality that needs to solve, e.g. how the profit is distributed among the drivers, this may change how driver earn.