Cannot revoke permissions or drop user in PgSQL AWS RDS
I was running into similar issue on Postgres RDS. This was the workaround I used to reassign objects from one user to another and then drop user. For you it would go like:

-- logged in as the rds_superuser role you defined when you created your RDS instance
CREATE ROLE change_owner LOGIN;
GRANT asd TO change_owner;
GRANT foo_migration TO change_owner;

-- log out of current user and log in as `change_owner`
REASSIGN OWNED BY asd TO foo_migration;

-- log out of current user and log in as rds_superuser role
DROP USER asd;