Discount Transaction Total, to Even $ amount

Hello,

I have a customer that wants to be able to set the total transaction price. For example, if all the items add up to $78.99, they want to be able to set the final price for the transaction (discount), to say $70 even.

We would then need that price broke down to a sub-total & tax amount (4.20 tax & 65.80 sub-total). So the discount would just be the difference between the initial total & the discounted total ($8.99 in the example).

I've looked at the options but don't know the best way to get it done - maybe thru an "internal html" that just prompts user for the transaction total, then runs script to update the transaction amounts.

I just don't quite know how to hook this into the system, and write it back properly.

Thanks for any help, Jon Farley