trade MacBook Air when buying iPad Pro

Yes, it is possible, however, you are not going to be able to do that directly through the Apple website. I believe you probably have discovered this and therefore you are asking the question here.

If you choose to complete this transaction online then visit ➔ Apple Trade-in Website. Here you would start the trade-in process of your MacBook Air, agree to the amount offered and accept the Apple Gift Card of the agreed value. The Gift Card will be issued to you via email once Apple receives your device and inspects it. You can then use the Gift Card to make the purchase for your new iPad Pro either through Apple.com or at the Apple Store. Details are in the link above.

For fastest results, you can take your MacBook Air to the nearest Apple Store and get an instant offer. If you agree on the offer, then you will be issued a Gift Card instantly. You can then use that Gift Card value to purchase your new iPad Pro while you are at the Apple Store.

Axel F.