Sounds like another failure in the making
Let me sum up the indicators :
- the exact time
- the rough location
- the force of the bump as measured by the accelerometer
- various other clues
Time is maybe the indicator with the most probability of exactitude, except that there is no guarantee that the two phones have the exact same time. So it'll be the exact same time within a given number of many milliseconds.
The location is charted probably following triangulation. Without other reference, you could be giving your cash to anybody in a 10-yard radius.
The force of the bump. Okay, one question : how accurate are those accelerometers ? Two random phones have what probability of containing similarly-accurate accelerometers ? Is this really needed ?
Frankly, I'd be more inspired by a solution that said "place the two phones together within 3 inches of each other and then authorize the transaction manually - the phones will sense their closest neighbor and, when authorized, complete the transfer".
In that kind of solution, there is no timestamp, no hazardous evaluation of location, no worry about someone else butting in. It's just the phone that is closest to yours, detected by BlueTooth or something similar. That would be just about as reliable and fool-proof as you can get.