Have you tried aggregating the bills in isolation from the rest of the query? Here's one way which looks clean and performs quite well:

SELECT

p.period

,m.clnt_matt_code AS [Client Code]

,m.matter_name

,o.offc_desc

,'Insolvency' AS matter_code

,ISNULL(l.Merged_Source_of_Work_Lender_Desc,k.name) AS Bank

,CASE WHEN m.matter_code NOT IN ('IBR','ISR','BNK','ADV')

THEN ISNULL(mins.original_estimated_fee,0) + ISNULL(mins.original_contingent_fee,0)

ELSE 0

END AS [Original Fee]

,isnull((

Select top 1 'Y'

from bo_live5.dbo.hba_source_bus AS ii

where ii.matter_uno=i.matter_uno and ii.comments LIKE '%walk-in%'

),'N') AS [Walk In],

em.employee_name,

isnull(x.billings_to_date, 0) AS billings_to_date

FROM bo_live5.dbo.hba_source_bus AS i

INNER JOIN bo_live5.dbo.cxa_folder_object AS j ON i.fo_uno = j.fo_uno

INNER JOIN bo_live5.dbo.hbm_name AS k ON k.name_uno = j.object_uno

LEFT JOIN BO_Custom.dbo.Source_of_work_Lender l ON l.Original_Source_of_Work_Lender_Desc = k.name --improtu look up for banks that have merged

INNER JOIN bo_live5.dbo.hbl_name_class AS h ON k.name_class_code = h.name_class_code

INNER JOIN bo_live5.dbo.hbm_matter AS m ON m.matter_uno = i.matter_uno

INNER JOIN bo_Live5.dbo.hbm_PERSNL AS em ON em.empl_uno = m.resp_empl_uno

--INNER JOIN bo_Live5.dbo.BLT_BILLM AS bi ON bi.matter_uno = m.matter_uno

CROSS APPLY (

SELECT billings_to_date = sum(bi.PROGRESS_BIL + bi.HARD_BIL + bi.SOFT_BIL + bi.FEES_BIL + bi.OAFEE_BIL + bi.RETAINER_BIL + bi.PREMDISC_BIL - bi.CREDIT_TOT)

FROM bo_Live5.dbo.BLT_BILLM bi

WHERE bi.MATTER_UNO = m.MATTER_UNO

) x

INNER JOIN bo_live5.dbo.hbl_office AS o ON o.offc_code = m.offc

INNER JOIN bo_live5.dbo.hba_offc_prof c ON c.offc = o.offc_code

AND LEFT(c.prof,2) IN (SELECT FieldName FROM dbo._000007_SplitList(@Profit_Center,',' ))

INNER JOIN CTE_Mins AS mins ON mins.matter_uno = m.matter_uno

LEFT JOIN bo_live5.dbo.glm_period_end AS p ON m.open_date BETWEEN p.per_begin_date AND p.per_end_date + '23:59:59'

WHERE ....................(Intentionally removed to reduce code)

--GROUP BY p.period

--,m.clnt_matt_code

--,m.matter_name

--,o.offc_desc

--,ISNULL(l.Merged_Source_of_Work_Lender_Desc,k.name)

--,m.matter_code

--,mins.original_estimated_fee

--,mins.original_contingent_fee, i.matter_uno ,em.employee_name