SQL Server 2019

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declare @MyTable TABLE(ClientId int, RecNo int, PaidAmt decimal(12, 2), PaidBefore decimal(12,2),
    ReimbToDate decimal(12,2), CanReimb decimal(12,2), BalBeforeReimb decimal(12, 2), 
    ReimbAmt decimal(12, 2), BalAfterReimb decimal(12, 2) )

insert into @MyTable (ClientId, RecNo, PaidAmt, PaidBefore, ReimbToDate, CanReimb, BalBeforeReimb ) select 1, 1, 100, 0, 0, 0 ,250
insert into @MyTable (ClientId, RecNo, PaidAmt, PaidBefore, ReimbToDate) select 1, 2, 400, 0, 0
insert into @MyTable (ClientId, RecNo, PaidAmt, PaidBefore, ReimbToDate) select 1, 3,-200 ,0 ,0

insert into @MyTable (ClientId ,RecNo ,PaidAmt ,PaidBefore ,ReimbToDate ,CanReimb,BalBeforeReimb) select 2 ,1 ,900 ,0 ,0 ,0 ,3000
insert into @MyTable (ClientId ,RecNo ,PaidAmt ,PaidBefore ,ReimbToDate) select 2 ,2 ,300 ,0 ,0
insert into @MyTable (ClientId ,RecNo ,PaidAmt ,PaidBefore ,ReimbToDate) select 2 ,3 ,1600 ,0 ,0
insert into @MyTable (ClientId ,RecNo ,PaidAmt ,PaidBefore ,ReimbToDate) select 2 ,4 ,-900 ,900 ,900
insert into @MyTable (ClientId ,RecNo ,PaidAmt ,PaidBefore ,ReimbToDate) select 2 ,5 ,-300 ,300 ,300
insert into @MyTable (ClientId ,RecNo,PaidAmt,PaidBefore,ReimbToDate) select 2,6,750,0,0
insert into @MyTable (ClientId ,RecNo,PaidAmt,PaidBefore,ReimbToDate) select 2,7,625,0,0
insert into @MyTable (ClientId ,RecNo,PaidAmt,PaidBefore,ReimbToDate) select 2,8,125,625,625

select ClientId,RecNo,PaidAmt
,PaidBefore,CanReimb,BalBeforeReimb
,ReimbAmt=case when CanReimb<BalBeforeReimb then CanReimb else BalBeforeReimb end,
BalAfterReimb=BalBeforeReimb-ReimbAmt
from (
		select ClientId,RecNo,PaidAmt,PaidBefore,CanReimb,ReimbAmt,case when  RecNo<>1 then 
		LAG (BalBeforeReimb) over (partition by ClientId order by ClientId) else 0 end as BalBeforeReimb

from @MyTable
)a

ClientId	RecNo	PaidAmt	PaidBefore	CanReimb	BalBeforeReimb	ReimbAmt	BalAfterReimb
1	1	100.00	0.00	0.00	0.00	0.00	null
1	2	400.00	0.00	null	250.00	250.00	null
1	3	-200.00	0.00	null	null	null	null
2	1	900.00	0.00	0.00	0.00	0.00	null
2	2	300.00	0.00	null	3000.00	3000.00	null
2	3	1600.00	0.00	null	null	null	null
2	4	-900.00	900.00	null	null	null	null
2	5	-300.00	300.00	null	null	null	null
2	6	750.00	0.00	null	null	null	null
2	7	625.00	0.00	null	null	null	null
2	8	125.00	625.00	null	null	null	null