Tracking brick and mortar leads

Hi all

I'm kinda new to offline marketing, but I have an idea that I think could make a nice profit, supplying clients/leads to some of the local brick and mortar businesses around here.

The only problem is... how do I track it? no websites are involved.

The owners could easily "adjust" the numbers to their favor ("what? you only gave us 1 customer, not 150.. take 2 bucks!" and such things that would be hard for me to prove)

So... any ideas?

Thanks!