Try this.

DECLARE @BOM TABLE

(

ItemIDINT

,neededQuantityfloat

)

INSERT INTO @BOM (ItemID, neededQuantity)

SELECT 1, 10

UNION ALL SELECT 2, 10

UNION ALL SELECT 3, 5;

DECLARE @WhareHouseMovement TABLE

(

ItemIDINT

,Quantityfloat

,DateDATETIME

)

INSERT INTO @WhareHouseMovement (ItemID, Quantity, Date)

SELECT 1, 10, '2015-03-01'

UNION ALL SELECT 1, -4, '2015-03-06'

UNION ALL SELECT 1, -4, '2015-03-08'

UNION ALL SELECT 1, 6, '2015-03-20'

UNION ALL SELECT 1, 12, '2015-03-24'

UNION ALL SELECT 2, 6, '2015-03-01'

UNION ALL SELECT 2, 6, '2015-03-04'

UNION ALL SELECT 2, -10, '2015-03-15'

UNION ALL SELECT 2, 8, '2015-03-20'

UNION ALL SELECT 3, 10, '2015-03-05'

UNION ALL SELECT 3, -4, '2015-03-15'

UNION ALL SELECT 3, -2, '2015-03-25'

UNION ALL SELECT 3, 6, '2015-03-26';

with dts as (

select date, b.ItemId, b.neededQuantity

from (select distinct date from @WhareHouseMovement ) x

cross join @bom b),

avl as (

select dts.ItemId, dts.date, neededQuantity - isnull(q,0) ds

from dts

outer apply ( select sum(Quantity) q from @WhareHouseMovement s

where s.ItemId = dts.ItemId and s.Date <= dts.date

) x

)

select min(date) readytomnfc

from (

select date, max(ds) mds

from avl

group by date

having max(ds) <=0 ) z

Concerning perfomance you may want to replace triangular join which calculates runnig totals with quirky update.