SQL Server 2016

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SELECT @@version;

DECLARE @ThresholdAmount NUMERIC(10, 2);

SET @ThresholdAmount = 20.00;

CREATE TABLE Transactions
(
    TransactionID INT
  , CustomerID INT
  , TransactionDate DATE
  , TransactionType VARCHAR(6)
  , TransactionAmount NUMERIC(10, 2)
);

INSERT INTO Transactions
VALUES
(1, 1, '2020-10-01', 'Credit', 10.25)
, (2, 1, '2020-10-02', 'Credit', 11.50)
, (3, 1, '2020-10-02', 'Debit', -13.25)
, (4, 2, '2020-10-01', 'Credit', 14.22)
, (5, 2, '2020-10-02', 'Debit', -50.75)
, (6, 2, '2020-10-04', 'Credit', 12.85)
, (7, 2, '2020-10-07', 'Debit', -4.53)
, (8, 3, '2020-10-02', 'Credit', 12.85);

SELECT
         txn.TransactionID
       , txn.CustomerID
       , txn.TransactionDate
       , txn.TransactionType
       , txn.TransactionAmount
FROM     Transactions txn
WHERE    EXISTS (
                    SELECT
                             txn1.CustomerID
                           , SUM(txn1.TransactionAmount) AS [TransactionAmoutTotal]
                    FROM     Transactions txn1
                    WHERE    txn1.TransactionType = 'Credit'
                         AND txn1.CustomerID      = txn.CustomerID
                    GROUP BY txn1.CustomerID
                    HAVING   SUM(txn1.TransactionAmount) >= @ThresholdAmount
                )
     AND EXISTS (
                    SELECT
                             txn2.CustomerID
                           , SUM(txn2.TransactionAmount) AS [TransactionAmoutTotal]
                    FROM     Transactions txn2
                    WHERE    txn2.TransactionType = 'Debit'
                         AND txn2.CustomerID      = txn.CustomerID
                    GROUP BY txn2.CustomerID
                    HAVING   SUM(txn2.TransactionAmount) <= (@ThresholdAmount * -1)
                )
ORDER BY txn.CustomerID
       , txn.TransactionType
       , txn.TransactionDate;

WITH txn_cte (TransactionID, CustomerID, TransactionDate, TransactionType, TransactionAmount, TransactionAmountTotal) AS
(
    SELECT
          txn.TransactionID
        , txn.CustomerID
        , txn.TransactionDate
        , txn.TransactionType
        , txn.TransactionAmount
        , txn.TransactionAmountTotal
    FROM  (
              SELECT
                   TransactionID
                 , CustomerID
                 , TransactionDate
                 , TransactionType
                 , TransactionAmount
                 , SUM(TransactionAmount) OVER (PARTITION BY CustomerID, TransactionType) AS [TransactionAmountTotal]
              FROM Transactions
          ) txn
    WHERE ABS(txn.TransactionAmountTotal) >= @ThresholdAmount
)
SELECT
         txn_cte.TransactionID
       , txn_cte.CustomerID
       , txn_cte.TransactionDate
       , txn_cte.TransactionType
       , txn_cte.TransactionAmount
FROM     txn_cte
WHERE    EXISTS (
                    SELECT txn_cte1.TransactionID
                    FROM   txn_cte txn_cte1
                    WHERE  txn_cte1.TransactionType        = 'Credit'
                       AND txn_cte1.CustomerID             = txn_cte.CustomerID
                )
     AND EXISTS (
                    SELECT txn_cte2.TransactionID
                    FROM   txn_cte txn_cte2
                    WHERE  txn_cte2.TransactionType        = 'Debit'
                       AND txn_cte2.CustomerID             = txn_cte.CustomerID
                )
ORDER BY txn_cte.CustomerID
       , txn_cte.TransactionType
       , txn_cte.TransactionDate;

(No column name)
Microsoft SQL Server 2016 (SP2-CU12) (KB4536648) - 13.0.5698.0 (X64) Feb 15 2020 01:47:30 Copyright (c) Microsoft Corporation Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)

TransactionID	CustomerID	TransactionDate	TransactionType	TransactionAmount
4	2	2020-10-01	Credit	14.22
6	2	2020-10-04	Credit	12.85
5	2	2020-10-02	Debit	-50.75
7	2	2020-10-07	Debit	-4.53

TransactionID	CustomerID	TransactionDate	TransactionType	TransactionAmount
4	2	2020-10-01	Credit	14.22
6	2	2020-10-04	Credit	12.85
5	2	2020-10-02	Debit	-50.75
7	2	2020-10-07	Debit	-4.53