way number 1:
(ddr=downpayment+deposit+rebatecustomer) - 
(tildp=ddr) - truth in lending down payment
(treq=trade-payoff)  - trade equity
(negtreq=0) - negative trade equity
(?(treq<0)negtreq=0-treq) - if it's negative then negtreq will have a positive #
(?(treq>0)tildp=ddr+treq) - it's positive so add it to the downpayment

so you put tildp as your downpayment in the truth in lending box and under where it breaks out everything.
under your to: and for: section you would put something like:
(?(negtreq>0)negtreq)

way number 2:
(ddr=downpayment+deposit+rebatecustomer+trade-payoff) - 
(tildp=ddr)
(?(ddr<0)tildp=0)

so you put tildp as your downpayment in the truth in lending box and under where it breaks out everything.
under your to: and for: section you would put something like:
(?(ddr<0)0-ddr)