I want replace one value with another in XML value. What i need is to use a substring method:

UPDATE zamowienia_xml
SET tresc_xml.modify('
    replace value of (/zamowienie/profil_rabatowy/rabat/text())[1]
    with
    (
        if (1=1)
        then
            text(
                   substring(
                    "abcd",
                    1,
                    2)
                )
            )
        else
            text("0")
    )');

I simplify the sql, it is not making what it should, but it still has the same error. Why this doesn’t work?
The error is:

[S0001][9303] Line 9: XQuery [zamowienia_xml.tresc_xml.modify()]: Syntax error near 'substring', expected ')'.

If someone what to see full sql, here can check:

UPDATE zamowienia_xml
SET tresc_xml.modify('
    replace value of (/zamowienie/profil_rabatowy/rabat/text())[1]
    with
    (
        if (substring(
                (/zamowienie/profil_rabatowy/rabat/text())[1],
                string-length((/zamowienie/profil_rabatowy/rabat/text())[1]), 1) = "%"
           )
        then
            text(
                substring(
                    string((/zamowienie/profil_rabatowy/rabat/text())[1]),
                    1,
                    string-length((/zamowienie/profil_rabatowy/rabat/text())[1]) - 1
                )
            )
        else
            (/zamowienie/profil_rabatowy/rabat/text())[1]
    )');

Error is the same:

[S0001][9303] Line 12: XQuery [zamowienia_xml.tresc_xml.modify()]: Syntax error near 'substring', expected ')'.

I feel like i tried everything, but problem still exist.
And about the sql, what I want to achieve is to remove optional character “%” at the end.