Some ERP systems needs the order_number and not order_id attached

If you look at https://git.drupalcode.org/project/commerce/-/blob/8.x-2.x/modules/order... you'll see the default time for the order number to be assigned to the order is order placement. Therefore unplaced orders by default do not have the order number. Clearly, in your case, the merchant made the requirement that even provisional orders (carts) must have the order number assigned. For that reason, we have the following event inhttps://git.drupalcode.org/project/commerce_easy/-/blob/1.x/src/OrderBui... you could subscribe and change the reference that is added to Easy. That being said, you may also explore another path and set the reference only after the payment completes and order gets the number assigned (via standard event subscriber). In such case you may want to explore https://developer.nexigroup.com/nexi-checkout/en-EU/api/payment-v1/#v1-p...

TL;DR
No patch is needed to fulfill the requirements for the merchant you support. The module provides API for interacting with the payload. Otherwise use Nexi API for updating the reference after order number gets assigned.

Thank you again :)

Subscribe to the commerce_order.place.post_transition event, and update the the EASY payment reference id with the Nexi API