Deu 16:17 Every man shall give as he is able, according to the blessing of the LORD your God that he has given you.
2 Corinthians 8:12 For if the readiness is there, it is acceptable according to what a person has, not according to what he does not have.
2 Corinthians 9:6-7 The point is this: whoever sows sparingly will also reap sparingly, and whoever sows bountifully will also reap bountifully. Each one must give as he has decided in his heart, not reluctantly or under compulsion, for God loves a cheerful giver.
Money bestowed in charity, may to the carnal mind seem thrown away, but when given from proper principles, it is seed sown, from which a valuable increase may be expected. It should be given carefully. Works of charity, like other good works, should be done with thought and design. Due thought, as to our circumstances, and those we are about to relieve, will direct our gifts for charitable uses. Help should be given freely, be it more or less; not grudgingly, but cheerfully. While some scatter, and yet increase; others withhold more than is meet, and it tends to poverty. If we had more faith and love, we should waste less on ourselves, and sow more in hope of a plentiful increase. Can a man lose by doing that with which God is pleased? He is able to make all grace abound towards us, and to abound in us; to give a large increase of spiritual and of temporal good things. He can make us to have enough in all things; and to be content with what we have. God gives not only enough for ourselves, but that also wherewith we may supply the wants of others, and this should be as seed to be sown. We must show the reality of our subjection to the gospel, by works of charity. This will be for the credit of our profession, and to the praise and glory of God. Let us endeavour to copy the example of Christ, being unwearied in doing good, and deeming it more blessed to give than to receive. Blessed be God for the unspeakable gift of his grace, whereby he enables and inclines some of his people to bestow upon others, and others to be grateful for it; and blessed be his glorious name to all eternity, for Jesus Christ, that inestimable gift of his love, through whom this and every other good thing, pertaining to life and godliness, are freely given unto us, beyond all expression, measure, or bounds. (Henry)
The idea is, that God will bestow rewards in proportion to what is given. These rewards may refer to results in this life, or to the rewards in heaven, or both. All who have ever been in the habit of giving liberally to the objects of benevolence can testify that they have lost nothing, but have reaped in proportion to their liberality. This follows in various ways.
(1) in the comfort and peace which results from giving. If a man wishes to purchase happiness with his gold, he can secure the most by bestowing it liberally on objects of charity. It will produce him more immediate peace than it would to spend it in sensual gratifications, and far more than to hoard it up useless in his coffers.
(2) in reflection on it hereafter. It will produce more happiness in remembering that he has done good with it, and promoted the happiness of others, than it will to reflect that he has hoarded up useless wealth, or that he has squandered it in sensual gratification. The one will be unmingled pleasure when he comes to die; the other will be unmingled self-reproach and pain.
(3) in subsequent life, God will in some way repay to him far more than he has bestowed in deeds of charity. By augmented prosperity, by health and future comfort, and by raising up for us and our families, when in distress and want, friends to aid us, God can and often does abundantly repay the liberal for all their acts of kindness and deeds of beneficence.
(4) God can and will reward his people in heaven abundantly for all their kindness to the poor, and all their self-denials in endeavoring to diffuse the influence of truth and the knowledge of salvation. Indeed the rewards of heaven will be in no small degree apportioned in this manner, and determined by the amount of benevolence which we have shown on earth. (Barnes)
The problem is that we keep too much to ourselves buying things we put in front of giving to God and then we “give as we are able” afterwards. Giving should always be first in our minds before we spend money we don’t have. Be in the practice of setting aside both the tithe and offerings and benevolent giving before thinking about other stuff.