The total is calculated from the tax table, not the other way around. Beware, there are events that will cause the taxes to be recalculated, so any value you set is not safe from being automatically changed by the system.

Tnx from reply,
My script which is changing the value is on Accept (Postwrite event of the table).So when i check again the same AR Invoice number the value is only changed in (Taxable Amount field) tab but not in ->TaxDetail ->Tax Amount.

And I explained why that happens. You need to create an object for the tax table and update the value there, not the total on the header.
But really, the system should calculate taxes correctly. Fix that properly not via script.

Excuse me for the stupid question but when you says you need to create an object for the tax table and update the value there which Bus or table do you mean to use to achieve it ?

Look in the help files for File Layouts. The table names (and associated business objects) are listed in there.

C:\Sage\Sage 100 Standard\MAS90\Home\Lib\_repository\mas5.30.01.00\env\any\CommonProgramFiles\Sage\Common Components\Help\5.30\FileLayouts.chm

Tnx for the replays Kevin and Kent u are helping me alot and i`m rly appreciate your help!

A little more info- we have custom tax schedule in which i`m trying to make a change on SalesTaxAmt
As documentation says AR_InvoceTaxSummery to get access you need :
KPRIMARY:InvoiceNo+InvoiceType+ScheduleSeqNo+TaxCode
KTAXCODE:TaxCode+InvoiceNo+InvoiceType+ScheduleSeqNo

Set oInvoiceTaxSummary = oSession.AsObject(oSession.GetObject("AR_InvoiceTaxSummary_bus"))
retVal = oInvoiceTaxSummary.SetKeyValue("InvoiceNo", strInvoiceNumber)
retVal = oInvoiceTaxSummary.SetKeyValue("InvoiceType$", strInvoiceType)
retVal = oInvoiceTaxSummary.SetKeyValue("ScheduleSeqNo", strScheduleSeqNo)
retVal = oInvoiceTaxSummary.SetKeyValue("TaxCode$", strTaxCode)
retVal = oInvoiceTaxSummary.SetKey()

retVal = oInvoiceTaxSummary.SetValue("SalesTaxAmt", 1234.02)

'I tried with or without .Write() method
retVal = oInvoiceTaxSummary.Write()

I know this functionality is working because Sage has default Tax schedule and it work fine.I think Sage get little confuse because there are 2 fields SalesTaxAmt - AR_InvoiceHeader which is my default bus in this example and AR_InvoiceTaxSummary which also has SalesTaxAmt field and the value i`m trying to change.

Txn for the replays Kevin and Kent u are helping me alot guys and Im rly appreciate your help!

A little more info - we have custom tax schedule in which im trying to make a change on SalesTaxAmt
As documentation says AR_InvoiceTaxSummary to get access you need :
KPRIMARY: InvoiceNo+InvoiceType+ScheduleSeqNo+TaxCode
KTAXCODE: TaxCode+InvoiceNo+InvoiceType+ScheduleSeqNo

Set oInvoiceTaxSummary = oSession.AsObject(oSession.GetObject("AR_InvoiceTaxSummary_bus"))
retVal = oInvoiceTaxSummary.SetKeyValue("InvoiceNo$", "0100084")
retVal = oInvoiceTaxSummary.SetKeyValue("InvoiceType$", "IN")
retVal = oInvoiceTaxSummary.SetKeyValue("ScheduleSeqNo$", "000001")
retVal = oInvoiceTaxSummary.SetKeyValue("TaxCode$", "EX")
retVal = oInvoiceTaxSummary.SetKey()

retVal = oInvoiceTaxSummary.SetValue("SalesTaxAmt", 1234.02)
retVal = oInvoiceTaxSummary.Write()
I know that this functionality is working because Sage has default Tax schedule and it work fine.I think Sage get little confuse because there is 2 fields SalesTaxAmt - AR_InvoiceHeader which is my default bus and AR_InvoiceTaxSummary which also has SalesTaxAmt field and the value im trying to change

You are totally right about the part that system override the value of the field after me. I accidentally saw that code is work when i was added a last line of code which rise Sage exception...Does anyone knows a way to tell Sage not to do this..I tried with SetToReadOnly(SalesTaxAmt) after Write() method but no luck.My code start with entering fields in edit mode -

If oBusObj.EditState>0 Then

'code over here

End If

Why are you trying to override the tax amount?