Here is the SQL:
 
UPDATE Item
SET [Quantity]=[Quantity]+:Value
WHERE [ItemCode]:=PartNo
 
In the unit I wrote:

ParamByName('Value').AsCurrency:=Value;  (Value is Currency type)
ParamByName('PartNo').AsString:=PartNo;

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 Comments

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Problem was caused by engine not allowing for unknown types (parameters, which are only known at execution time) in certain expressions during the preparation of the query.

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 Resolution

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Fixed Problem on 4/4/2002 in version 3.09 build 1