I want to know if someone made a sinful promise to God, what happens then? Is the person to keep that promise still? Are we to keep our promises no matter what?
"Whoever has been born of God does not sin, for His seed remains in him; and he cannot sin, because he has been born of God" (I John 3:9). John lets us know that for us to be children of God, we are not to sin. Sinning is contrary to the nature of a follower of Christ.
"If a man makes a vow to the LORD, or swears an oath to bind himself by some agreement, he shall not break his word; he shall do according to all that proceeds out of his mouth" (Numbers 30:2). If you make a promise, whether to God, or in any other agreement, you are to keep your word. "But let your 'Yes' be 'Yes,' and your 'No,' 'No.' For whatever is more than these is from the evil one" (Matthew 5:37).
What if you have made a promise to do something sinful? By promising to sin, you have already broken I John 3:9. When you sin, you have to repent to become right with God. "Repent therefore and be converted, that your sins may be blotted out, so that times of refreshing may come from the presence of the Lord" (Acts 3:19). To repent is to turn away from your sinful actions and start acting in a God-fearing manner. You cannot keep a promise to sin if you wish to repent; that would not be turning from your sins.
In order to straighten things out, you will need to apologize to any and all parties involved with the agreement you made. For example, if you had promised your boss that you would deceive the shareholders' meeting about the progress of your project, you need to contact your boss and tell him that you are not going to be able to keep that promise and that you are sorry for misleading him. If your promise was only between you and God, then you still need to apologize to God and ask His forgiveness in prayer.
And in the future, be cautious with your words.
"For we all stumble in many things. If anyone does not stumble in word, he is a perfect man, able also to bridle the whole body" (James 3:2).
"He who guards his mouth preserves his life, But he who opens wide his lips shall have destruction" (Proverbs 13:3).
"But I say to you that for every idle word men may speak, they will give account of it in the day of judgment. 'For by your words you will be justified, and by your words you will be condemned'" (Matthew 12:36-37).
If we made a promise to God, then does that mean we can ask Him to free us from the vow we have made? What if the person made a vow that from now on he is just going to buy food and clothes? Does it mean the person has to keep at this; otherwise, he will sin every time he buys something?
A person who makes a foolish vow that he cannot keep has already sinned. "Walk prudently when you go to the house of God; and draw near to hear rather than to give the sacrifice of fools, for they do not know that they do evil. Do not be rash with your mouth, And let not your heart utter anything hastily before God. For God is in heaven, and you on earth; Therefore let your words be few" (Ecclesiastes 5:1-2). You apologize to God for your sinful behavior (I John 1:9) and then think before speaking.