SQL Server 2019

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table tbl
(
	Reference	varchar(5),
	Debit		int,
	Credit		int
)
insert into tbl values
('FT120', 50,   NULL),
('FT120', NULL, 50),
('FT120', 50,   NULL),
('FT589', 100,  NULL),
('FT589', NULL, 100),
('FT780', 60, NULL);
6 rows affected

with trans as
(
	select	Reference, Debit, Credit,
		r = row_number() over (partition by Reference, Debit, Credit
					order by Debit, Credit)		
	from	tbl 
)
select	*,
	case when sum(isnull(Debit, 0) - isnull(Credit, 0)) over (partition by Reference, coalesce(Debit, Credit), r)
			= 0
	then 1
	else 0 
	end
from	trans;


Reference
Debit
Credit
r
(No column name)


FT120
null
50
1
1

FT120
50
null
1
1

FT120
50
null
2
0

FT589
null
100
1
1

FT589
100
null
1
1

FT780
60
null
1
0

Reference	Debit	Credit	r	(No column name)
FT120	null	50	1	1
FT120	50	null	1	1
FT120	50	null	2	0
FT589	null	100	1	1
FT589	100	null	1	1
FT780	60	null	1	0