Engines are run at the factory and that oil is drained. I'm one of those who has learned the hard way that you follow the manual. No second-guessing what viscosity oil to use or how soon to change it. The engineers at the factory are telling you to leave the oil in it long enough to ensure ring seating and other break-in factors to insure a long-lasting engine.
Does it have an oil filter? If it does, that's even more reason to not change it too soon. If on the outside chance there were some stray pieces of metal in the engine, they would be trapped in the filter within seconds of the engine's first start.
Does the tractor come from the dealer with oil in the engine? If so, you need to know exactly what it is - brand, viscosity and special features, if any. What I am concerned about here is if the dealer has used special 'break-in' oil in it, in which case he would be correct about the early change. However, the chances of that are slim. In any event, you need to know what's in it. Maybe they used an oil too 'heavy' or too 'light'. You need to know.
Do you have the ability to change the oil/filter yourself, or do you have to take it to a shop for that service? I'm wondering if the salesman is just blowing smoke to get a service job for the shop.
I'm trying to convince another guy on a car forum that he is causing an early 'dark oil' problem on a new turbo car by using oil that's too thick and changing it too early. I have the same car without turbo, and it took two oil changes of 5000 miles each before fuel mileage reached its peak, so I know that an extended break-in is normal for this vehicle.
The ONLY time an early change is indicated is if special 'break-in' oil is used. This oil contains extra zinc and other additives to prevent 'galling' and other wear during the very first start of a new engine, but it is not designed to last a full change. Typically, auto and lawn equipment manufacturers today never ship a product with break-in oil in the engine, because they have already tested the engine at the factory. Consequently, there no longer is the need for early oil changes.

Had SBC 406 rebuilt one time with the break in oil installed after about 300 miles I changed the oil. Should of waited twice that long for good cam run in. Got flat cam, learnt my lesson about changing oil too soon on new engine when break in oil installed. course that was 30 years ago. I sure technology has changed. Grim.....If it was me I would leave in till at least 25 hours. (if it makes you feel good you can just change the filter and top it back off). My conditions don't allow 50 hour oil changes. I change mine between 20 and 30 hours tops usually.

Thanks guys, just to answer your questions :
Yes, tractor (22HP, 42" cut) came with oil and filter in place ready to go., bought at Sears, was delivered, it was the show room floor guy who told me to change it after 5 hours, the manual says 50. I can easily change the oil & filter myself a-ok, done it many a time on older lawn tractors. If break-in oil is no longer use (not mentioned in manual) then maybe I'll split the difference and wait till 25 hours like suggested. Thanks again!