>>You can try
>>

>>SELECT Client.Numero FROM Client
>>  WHERE Client.NoEntity=@NoEntity AND Client.Fic_Date<@Fic_Date 
>> 	AND Client.Loss_Code=@Loss_Code AND Client.CType=@CType
>> 	AND EXISTS ( SELECT 1 FROM Invoice WHERE Client.Numero=Invoice.NoClient AND Invoice.Amount=@Amount)
>>

>
>Thanks, I ended up doing something like that last night. I will keep you inform later on if this approach resolves the issue.

If the potential for dirty reads is not an issue, you could add 'WITH (NOLOCK)' table hint to eliminate locks during the select

SELECT Client.Numero FROM Client WITH (NOLOCK)
  WHERE Client.NoEntity=@NoEntity AND Client.Fic_Date<@Fic_Date 
 	AND Client.Loss_Code=@Loss_Code AND Client.CType=@CType
 	AND EXISTS ( SELECT 1 FROM Invoice WITH (NOLOCK) WHERE Client.Numero=Invoice.NoClient 
                AND Invoice.Amount=@Amount)