Change iPad

If purchased directly from Apple, you have 14-days within which you can unconditionally return an unwanted device for exchange or refund. After expiry of 14-days, this opportunity is lost. As such, your proposal to return your iPad and exchange for a smaller/cheaper model - and receive the cost difference - isn’t available to you.

Trade-in may be an option - although trade-in is only available for previous generation models, not current models. Presumably you have a fifth or earlier generation iPad Pro 12.9” which should qualify; he available trade-in value will be considerably less than the price paid. You can obtain an estimated trade-in value here:

Apple Trade In - Apple

Your remaining option is to consider selling your iPad Pro privately - although to make a sale, potential buyers will likely expect to pay much less than the “as new” cost of the iPad. You might expect to sell your iPad for more than the trade-in value, but somewhat less than purchasing a reconditioned identical model from Apple.

best you can do is to try

Apple Trade In - Apple

though if it's the newest model they don't offer trade in in which case your best bet is to sell it on your own