"Treasures gained by wickedness do not profit" (Pr 10:2 ESV)
"If the first portion of the dough offered is holy, then the whole batch is holy" (Ro 11:16 NET)
Money itself is neutral. It is neither good nor evil (in the moral sense. Having money is a "good" thing in the blessing sense, obviously!) It is the love of money that is a root of all kinds of evil (1Ti 6:10).
However, does this mean that all money is equal? The obvious answer from a purely natural outlook is "Yes, of course!" The £20 note in my pocket it identical in value to the one in yours. It does not entitle me, in a fiscal way, to any more or less than it entitles you. But when we look into God's word, we see that it is God who gives wealth, and it is also he who gives or deprives the ability to enjoy or benefit from that wealth. One man may have much wealth, and yet that money brings him no happiness, in fact it may actually bring him misery, whereas another man with much less may find great contentment and blessing in the money God has given him (Ecc 5:12).
We also see in God's word the principle that a man's labours, and the fruits of his labours, can be blessed by the Lord, or they can be under God's judgement (Deuteronomy 28). So we may have much and yet because of the Lord's judgement it may yield us little in return, or we may have little, but because of the Lord's favour towards us it can yield much. The obvious principle is that it is always better to have a little money with God's blessing, than a whole heap of cash without (Pr 15:16).
It is something to bear in mind, next time you are tempted to fiddle your expenses, omit things from your tax return, install unlicensed software on your computer, or the myriad of other ways we might be swayed to "get a little back" in ways that "don't really matter." It does matter. What will bless you more, saving a few pounds, or the blessing of the Lord on what you have already? Treasures gained by wickedness do not profit!
If you thought I was going to bring this back to the issue of tithing... you were right! By now you should be able to join the dots, but just in case... If money that is robbed from man can never profit, how much less will money that has been robbed from God!? I may have an extra hundred pounds or so in the bank at the end of the month, but what good will that do me unless the Lord blesses me to enable me to prosper from it or to find enjoyment in it. But there is another promise here (I know I'm taking it out of context, but so was Paul!) If the first portion is offered as holy to the Lord, then the rest of the batch is made holy. If we tithe and acknowledge that the first part of our income is holy to the Lord, then the promise of God is that the rest will be blessed. It is a principle of God that he invites us to test him on (more on this another day.)