You are right Ann, that Abraham was stopped from sacrificing Isaac and that Jephthah sacrificed his daughter but the circumstances are completely different and does not show any favoritism on God's part.
God ordered Abraham to sacrifice his son in order to test him. God knew he would never have Abraham go through with it but wanted to see if Abraham was willing to follow God's will, even if it meant sacrificing his beloved son.
Jephthah made a foolish vow all on his own: that if God were to give the Ammonites over to him, he would sacrifice the first thing that ran out of his front door upon returning home. God did not say he needed to make this vow or that he wouldn't have given the Ammonites over to him anyways. Nor does it say what would have happened to Jephthah had he not followed through with sacrificing his daughter. In this story, God was not testing Jephthah, he commited that atrocity all on his own.
People do foolish things in the Bible all the time, that are not sanctioned by God. For example, when Saul and his army were going to fight the Philistines, Saul made a very foolish vow: 1 Samuel 14:24-30
24 Now the men of Israel were in distress that day, because Saul had bound the people under an oath, saying, "Cursed be any man who eats food before evening comes, before I have avenged myself on my enemies!" So none of the troops tasted food.
25 The entire army [d] entered the woods, and there was honey on the ground. 26 When they went into the woods, they saw the honey oozing out, yet no one put his hand to his mouth, because they feared the oath. 27 But Jonathan had not heard that his father had bound the people with the oath, so he reached out the end of the staff that was in his hand and dipped it into the honeycomb. He raised his hand to his mouth, and his eyes brightened. [e] 28 Then one of the soldiers told him, "Your father bound the army under a strict oath, saying, 'Cursed be any man who eats food today!' That is why the men are faint."
29 Jonathan said, "My father has made trouble for the country. See how my eyes brightened [f] when I tasted a little of this honey. 30 How much better it would have been if the men had eaten today some of the plunder they took from their enemies. Would not the slaughter of the Philistines have been even greater?"
Jonathan does not end up getting killed/sacrificed by his father because his men stand up for him.
43 Then Saul said to Jonathan, "Tell me what you have done."
So Jonathan told him, "I merely tasted a little honey with the end of my staff. And now must I die?"
44 Saul said, "May God deal with me, be it ever so severely, if you do not die, Jonathan."
45 But the men said to Saul, "Should Jonathan die—he who has brought about this great deliverance in Israel? Never! As surely as the LORD lives, not a hair of his head will fall to the ground, for he did this today with God's help." So the men rescued Jonathan, and he was not put to death.
