The Torah describes (starting in Bamidbar 5:11) the process of a husband warning his wife not to be secluded with a certain man. Then if she is secluded with that man for a certain amount of time and witnesses (or the husband) see the seclusion she is eligible to go through the sotah process. I would like to know if the husband at any point can undo his initial warning so that the woman's seclusion with the second man cannot precipitate the sotah process.
yes, and it is retroactively invalidated, in fact he can even undo the warning done on his behalf by the court. see Noda BiYehuda Tinyana Even HaEzer 159