Forget the brand, or even that it is a tractor. Consider finance incentives and warranty on this "item". The actual depreciation is more like 10%, the balance is fighting the economics of subsidized financing and warranty value.
A used tractor, even slightly used, can no longer be sold with the 0% financing, so calculate the amount one would pay at the normal non-sponsored rate of 9.4% as compared to 0% for 36 months, or figure the 60 month rate comparison if you prefer. On the 36 month comparison, the new tractor buyer will save $3600 in interest over a standard rate. So if the dealer tries to resell your used tractor for $19,900, ($23500 less $3600), the new buyer could buy a brand new one for the same monthly payment and have a new tractor. So we know this item needs to be less than $19,900 to compete with new. Now take warranty; this has a few months left on warranty. Figure a new item warranty is worth $1000, which is a made up number, but I'll use it just the same. Now the tractor needs to sell for $18,900 to break even with new. Now factor in that it is used, has a few scratches I assume, tires are worn some, clutch might be a third gone, etc. Let's take off just $2000 for almost three years wear and tear. Now we are at $16,900. Let's say the dealer does a full service and then wants to make 5%. That puts a trade-in value around $16,000?
Of course there are other ways to look at this. A cash buyer may not think of the value of the money borrowed and may just see a new one at $23,500 and a slightly used one might fetch $19,500.
In summary, whether this is a new pickup or a new tractor, as long as the manufacturers are offering subsudized rates worth thousands and long warranties, used items will take a 30% hit right off the bat. Keep it another year or two and it'll still be worth what it is now.
As a practical matter, make him an offer. Good trade-ins are great to have on the lot and he should want it.
Does this make sense?
