There is no need to change a written offer when a trade is added, beyond counting the trade as part of the buyer's contribution - the OTD price remains the same. As for the scenario you present in which the dealership refuses to honor an offer they have just made: that happens all the time, unfortunately, and is usually called a "mistake." Again, it's unfortunate that holding back the trade should be done, and it quite possibly isn't necessary with every dealership, but there are just too many dishonest dealers out there who would take advantage of the situation. If a prospective buyer/lessee feels that they have a good, honest, established relationship with a dealership then they can perhaps skip what I suggest. Given the rapid staff turnover rate at dealerships, though, I wouldn't do that unless all of the same people (owner, salesperson, finance manager) were still there.
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
BAFX OBDII Dongle
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.