Adding the customer id to your returning clause and using update..from could help:
with data as (
delete from orders
where customer_id = <customer id>
returning customer_id, price
), total as (
select customer_id, sum(price) as total_price
from data
group by customer_id
)
update paymentdetail
set temp_credit = temp_credit + total.total_price
from total
where customer_id = total.customer_id
You could skip the "total" cte and just update the same rows repeatedly. I'm not sure if the same row being repeatedly updated in the same statement creates additional row versions or just updates the existing one.
...CTE’s act as optimisation fences.
It might be worth noting PG12 changes that behavior in simple cases where the CTE is not recursive, not referenced more than once, and is side-effect free.
