Delegation of attestation

Maybe you could create a WhereClause filter on the delegation candidates endpoint
/portal/delegations/UID_PersonReceiver/candidates
(p.s. this solution is not exclusive for delegations of type attestation)

Administration Portal > Configuration: Web Portal (click on: three dots icon)
Create configuration key
Select where to add the configuration key: API method configuration
Enter the API method name, replacing slashes with underscores, for example "itshop_cart".
Name of the new configuration key: delegations_UID_PersonReceiver_candidates
[Create]

[Search] delegations_UID_PersonReceiver_candidates
API method configuration / delegations_UID_PersonReceiver_candidates / Filter condition
1 = 0 dummy query to see if it's working

Use '%useruid%' to get the UID of the person logged in: UID_Person = '%useruid%'

For your use case...

uid_department IN (
		SELECT uid_department
		FROM person
		WHERE uid_person = '%useruid%'
		)