what if i sold my ipad after i agreed to the trade in credit

I purchased a new 12.9 iPad pro (most recent version) and I opted to trade in my older iPad pro (3rd generation 256gb). However, in the meantime I had a someone wanting to purchase the trade in device. I wasn't sure how this would affect the trade in and etc. I know that my monthly payment would go up but, I'm just trying to determine if this can be done or is it frowned upon since I already agreed to the trade in credit. Thanks for your help.