Sergey-

>I don't know why it's happend but there is a simplier code that does the same
>

>SELECT TABLE_A
>SCAN FOR Amount<>0
>      STORE Number TO xNbr
>      SELECT TABLE_B
>      LOCATE FOR Field1B=xNbr
>      * To be safe you should check here if the record is found or not
>      REPLACE Field2B WITH TABLE_A.Amount
>ENDSCAN
>

Instead of selecting, why not just:

IF !EOF("Table_B")
REPLACE Field2B WITH TABLE_A.Amount IN Table_B
ENDIF