Your husband can, should he choose to do so, trade-in his old iPad at a later date.
The trade-in, if accepted as fault free, will be settled by way of a non-expiring Apple Gift card - that can be used against a future purchase.
When (if) your husband is ready to do so, he can either trade-in at an Apple Store, or initiate the trade-in online:
Apple Trade In - Apple
Many choose to perform the trade-in at an Apple Store, as the assessment and final value can be agreed and the transaction finalised. By contrast, an online trade-in requires the device to be returned to Apple's trade-in partner for assessment - leaving some doubt at to the final value until the device has been subsequently assessed.
Before parting with the old iPad, ensure that it has been properly prepared:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support