DATEADD( dd, 5 - DATEPART( dw, GETDATE()), GETDATE())

--===== Conditionally drop the test table to make reruns in SSMS easier.
     IF OBJECT_ID('tempdb..#TestTable','U') IS NOT NULL
        DROP TABLE #TestTable
;
GO
--===== Populate the table with test data.
     -- This is not a part of the solution.
 SELECT TOP 100
        DueDate = ABS(CHECKSUM(NEWID())) % DATEDIFF(dd,'2000','2020') + CAST('2000' AS DATETIME),
        PayDate = CAST(NULL AS DATETIME)
   INTO #TestTable
   FROM sys.all_columns ac1
  CROSS JOIN sys.all_columns ac2
; 
--===== This calculates the pay-on date for each date
     -- and stores it in the same table.
     -- This is a solution that doesn't depend on DATEFIRST.
 UPDATE #TestTable
    SET PayDate = DATEADD(dd,DATEDIFF(dd,4,DueDate+6)/7*7,4)
;
--===== Display the results for verification.
     -- This is not a part of the solution.
 SELECT DueDate, DATENAME(dw,DueDate),
        PayDate, DATENAME(dw,PayDate)
   FROM #TestTable
;