The value of X is determined only as Syr Faren's triggered ability resolves. Once that happens, the value of X won't change later in the turn even if Syr Faren's power changes.

If Syr Faren leaves the battlefield before its triggered ability resolves, use its power as it last existed on the battlefield to determine the value of X.

If Syr Faren's power is negative as its triggered ability resolves, X is considered to be 0.