And he who supplies seed to the sower and bread for food, will supply and multiply your seed for sowing, and increase the fruits of your righteousness, you being enriched in everything unto all liberality, which works through us thanksgiving to God.'
And the consequence will be that the One Who on a continuing basis supplies seed to the sower (which supply is evidence of previous blessing) and bread to the eater, will be sure to multiply their ‘seed' (their wealth) so that they may have more to sow and can give even more. He will ‘increase the fruits of their righteousness', that is, in context, that he will provide them with more and more benefit (fruits) for the continual carrying on of their righteous behaviour as revealed in their generous giving. They will be enriched in everything so that they can be even more liberal.
Note how Paul's promise is not that they will themselves become personally prosperous, but that they will be provided with the means to be able to give more and more, and to be more and more generous. Unlike many today who promise to donors that if they give they will thereby become personally prosperous, Paul is not concerned with the growth of their personal wealth, but with the growth of their spiritual generosity. He wants them to abound, not their wealth.
The thought is taken from Isaiah 55:10 (compare also Hosea 10:12 LXX for ‘the fruits of their righteousness') where the idea is of the carrying forward of God's final purposes as His word goes forth to accomplish His will, so that Paul is not only making a general practical application but showing in these results the fulfilment of God's eternal plan. As a result of their generous giving they will be caught up even more in the continual going forward of God's saving process.
‘Which works through us thanksgiving to God.' And the result of all their liberality will result through the bearers of their gifts (‘us') in thanksgiving to God by those who receive them. Thus are they contributing to increased worship of God.