Although warned against making rash vows, the Israelite leaders did just that in making a treaty with the Gibeonites. The leaders came clean when they realised what had happened, but they were not willing to break a vow:
9:19-21 But the leaders replied, ‘Since we have sworn an oath in the presence of the Lord, the God of Israel, we cannot touch them. This is what we must do. We must let them live, for divine anger would come upon us if we broke our oath. Let them live.’ So they made them woodcutters and water carriers for the entire community, as the Israelite leaders directed.
Although they leaders failed to ask God before they made the treaty, there is no suggestion that God was displeased with them for making it in what we would call ‘good faith’.
That said, we aren’t told whether there was a sin offering, as required by Leviticus 5:4; but the Achan incident really had put the fear of God into the Israelites, so it is likely the leaders made sure of it.