SQL query question - tricky one
You have to build a bridge, which you should already have, so you should think about your design
The only useful and not redundant information that has the table payment is LFN and to be honest put this into the payment_positions, every information you want you can get from payment_positions then:
CREATE TABLE [payment] (
  [LFN] INTEGER,
  [OUR_RECNO] INTEGER,
  [TOTAL] FLOAT
);

INSERT INTO [payment]
  ([LFN], [OUR_RECNO], [TOTAL])
VALUES
  ('1234', '36622016', '71.89');
GO 

1 rows affected

CREATE TABLE [payment_positions] (
  [OUR_RECNO] INTEGER,
  [CUSTOMER] INTEGER,
  [TOTAL] FLOAT
);

INSERT INTO [payment_positions]
  ([OUR_RECNO], [CUSTOMER], [TOTAL])
VALUES
  ('36617800', '2566303', '47.34'),
  ('36622016', '2566303', '10.51'),
  ('36625936', '2566303', '14.04');
GO

3 rows affected

SELECT [CUSTOMER],[OUR_RECNO] 
FROM [payment_positions] 
WHERE [OUR_RECNO] IN (SELECT [OUR_RECNO] FROM [payment] ) 

CUSTOMER | OUR_RECNO
-------: | --------:  
 2566303 |  36622016

SELECT r.[LFN], ac.[OUR_RECNO], ac.[CUSTOMER], ac.[TOTAL]
FROM [payment] AS r
INNER JOIN 
(SELECT [CUSTOMER],[OUR_RECNO] FROM [payment_positions] WHERE [OUR_RECNO] IN (SELECT [OUR_RECNO] FROM [payment] )) AS t1
ON r.[OUR_RECNO] = t1.[OUR_RECNO]
INNER JOIN [payment_positions]  AS ac
ON ac.[CUSTOMER] = t1.[CUSTOMER]

 LFN | OUR_RECNO | CUSTOMER | TOTAL
---: | --------: | -------: | ----:
1234 |  36617800 |  2566303 | 47.34 
1234 |  36622016 |  2566303 | 10.51 
1234 |  36625936 |  2566303 | 14.04

db&lt;&gt;fiddle here