The Cash Discount Percent can be found in Setup > Setup POS & POS Devices. It is the inverse of the Non Cash Payment Percent using the formula NCP / 1+NCP using the numerical value.

For example, if Non Cash Payment is 3.75%, the Cash Discount is .0375 / 1 + .0375 = .0361... or 3.61%. 

When a ticket is paid in Cash, the 3.61% discount is applied to the Services, Products, Classes and Packages on the ticket and the tax will be updated as well.