Question: "What should be done if a husband and wife disagree on tithing / how much to give?"
Answer: When a husband and wife disagree on "tithing" or on how much to give to the local church and other ministries, strife can result. First, it is important to understand that Christians under the New Covenant are under no obligation to tithe 10% of their income. God instituted tithing to Israel in the Old Testament economy. The tithe was a practice even before the law was given (Genesis 14:20), and Leviticus 27:30 says that the people were to tithe of the land, seed or fruit of the trees for it all belongs to the Lord. In Deuteronomy 14:22, Moses relates to the people that God says, "Be sure to set aside a tenth of all that your fields produce each year." The people of Israel were to bring a tenth of all their increase and give it back to the Lord. The tithe supported the tabernacle and later the temple as well as the priesthood.
Today, our tithes and offerings are a love offering we give to God in thanksgiving for the blessings that we receive as His children. We are not under the law of the Old Testament economy but in the age of grace. Our tithes and offerings are a way to support God's work in our local churches as well as missionary endeavors.
When we give to the Lord, we are to give out of a cheerful heart. "Remember this: Whoever sows sparingly will also reap sparingly, and whoever sows generously will also reap generously. Each man should give what he has decided in his heart to give, not reluctantly or under compulsion, for God loves a cheerful giver" (2 Corinthians 9:6-7). Giving out of compulsion or giving in order to make up for something lacking will not profit us spiritually, nor will it bring blessing to the household.
In God's order, the husband and the wife are one (Mark 10:8). Ideally, the husband and wife should discuss their giving and come to a mutual agreement on both the appropriate amount and appropriate places to give based on biblical principles. If there is a disagreement about giving, the wife cannot usurp her husband's authority and give in his place or try to stop him from giving. In so doing, the wife takes the headship authority (Ephesians 5:22-33) upon herself, and that is outside of God's order. Wives are to walk in obedience to God's command and submit as unto Him (Ephesians 5:22). Likewise, husbands are to submit to God and to love their wives selflessly (Ephesians 5:22-33). A husband should prayerfully consider his wife's input and ultimately follow the leading of the Lord. If either spouse is an unbeliever, the same principles still hold. The husband, as the head of the family, bears final responsibility for decisions about giving.
Submission to God's order will bring a blessing and the grace to stand in faith. God has a way of getting things done, and we can confidently stand still and watch without taking it upon ourselves to right what we see as wrong. In 1 Samuel we find this eternal principle: "But Samuel replied, 'Does the LORD delight in burnt offerings and sacrifices as much as in obeying the voice of the LORD? To obey is better than sacrifice, and to heed is better than the fat of rams'" (1 Samuel 15:22).