For uploading assets you have to use the t code AS91 Which updates the asset ledger not the financial books The logic should be as follows You should first maintain the specifications for Takeover And enter last period in which depreciation is posted 

You should have a logic by which you should upload the assets through AS91 for additions it should not calculate accumulated depreciation Then after uploading you should pass financial entry through OASV Hope its clear in case of any doubt revert back .

nothing wrong with the data you have received from your client, please create a new GL account for upload like some 'upload clearing account' as a balance sheet account use this account for debit/credit for BDC and create a 2 more columns on your flat file one for debit/credit for each line and one with the new account eg: if data received is 40 - GL Account - Amount 50 - GL Account - Amount 50 - GL Account - Amount 

Then modify it with the new account created for 'GL upload clearing' as 

40 - GL Account - Amount - 50 - GL upload clearing account - Amount 50 - GL Account - Amount - 40 - GL upload clearing account - Amount 50 - GL Account - Amount - 40 - GL upload clearing account - Amount 

Same way create an account for customer and vendor open item upload too. if your upload goes well with all the posting required for the data your clearing accounts should balance in 0. 

Fixed Assets data migration is part of the cut over activities as journal balances are uploaded. The cut off date can be the financial year end date or mid year. Before you start the upload process, make sure the total values like APC, accumulated depreciation and depreciation for the year matches with the legacy GL balances.Steps for Fixed Assets data migration in SAPCheck customising for transfer dateAsset Accounting -> Preparing for Production Start Up -> Asset Data Transfer -> Parameters for Data Transfer ->

Date Specifications\ Specify Transfer Date/Last Closed Fiscal Year

Transfer date – this should be set to close to the date you migrate the assets. As AS91 is specifically for data migration, it expects that the capitalisation date for all the assets you migrate will be before the transfer date. Check this in table T093C.

Current fiscal year – also in T093C, check that this has been correctly set. Also check Number ranges have been maintained

Legacy Asset Master creationTransaction AS91 is for creating legacy asset master. You will be using LSMW to upload the file and create fixed assets. The takeover value screen will have Accumulated APC, Accu Ord Dep and Depreciation.

Accumulated APC = Original valueAccu Ord Dep = Accumulated depreciation till the end of last yearDepreciation = Depreciation for the year

When the assets are capitalised during the year, then on Asset Takeover Values, you have a button "transactions". Click the button and enter value date, transaction type 100 and amount (acquisition value).

Fixed Asset masters are created. After you have uploaded the fixed assets, check the report for Assets Balances and verify that the balances matches with what you have uploaded. Depreciation for the year may not match since the system is calculating planned depreciation.

GL balance upload Reset the asset reconciliation GL account in customising. This will allow

direct posting in the GL accounts Some recommend to use transaction OASV to upload the asset GL balances

as it directly allows posting to reconciliation account. . However I found when posting depreciation for the current year balances, cost centre field is not available. I would advise you to use transaction F-01 for asset balance upload with cut over date as your posting date.

The journal entry is

Debit ….. APCCredit  ….. Accumulated DepreciationDebit  …. Depreciation (year to date)Credit …. Data Take on (balancing figure)

Reset asset reconciliation GL account in customising to make it reconciliation account.

That completes fixed assets data migration.