Hi @Repetto Asensio, Lucas Nicolas ,
please take a look here: Transfer billing ownership of an MOSP Azure subscription to another account
If this is not working for you the best option might be to open a support case with Microsoft Azure support for clarification.
Azure Billing and Subscription support is offered by Microsoft by no cost and it's for free.
Source: Azure Support plans
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten