UPDATE m Set
POObligations = x.POObligations
FROM MDEP m
CROSS APPLY (
SELECT
POObligations = (SUM(PO_DX.QuantityOrdered * PO_DX.Price) - SUM(Distinct PO_MX.PaymentPosted))
* SUM(Distinct(cast(PO_MX.PrimarySplit as decimal(5,2))/100))
FROM PO_MX
INNER JOIN PO_DX
ON PO_DX.PONumber = PO_MX.PONumber
WHERE PO_MX.MDEP = m.MDEP
) p
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden